Campbell Biology

two alleles, the three genotypes will appear in the following Table 23.1  Conditions for Hardy-Weinberg Equilibrium 
proportions:

p 2 + 2pq + q 2 = 1 Condition Consequence if Condition Does Not Hold
1. No mutations
Expected Expected Expected 2. Random mating The gene pool is modified if mutations occur
frequency frequency frequency or if entire genes are deleted or duplicated.
of genotype of genotype of genotype 3. No natural
selection If individuals mate within a subset of the
CRCR CRCW CWCW population, such as near neighbors or close
4. E xtremely large relatives (inbreeding), random mixing of
Note that for a locus with two alleles, only three genotypes population size gametes does not occur and genotype
are possible (in this case, CRCR, CRCW, and CWCW). As a result, frequencies change.
the sum of the frequencies of the three genotypes must equal 5. No gene flow
1 (100%) in any population—regardless of whether the popu- Allele frequencies change when individuals
lation is in Hardy-Weinberg equilibrium. The key point is with different genotypes show consistent
that a population is in Hardy-Weinberg equilibrium only if differences in their survival or reproductive
the observed genotype frequency of one homozygote is p 2, the success.
observed frequency of the other homozygote is q 2, and the
observed frequency of heterozygotes is 2pq. Finally, as sug- In small populations, allele frequencies
gested by Figure 23.8, if a population such as our wildflowers fluctuate by chance over time (a process
is in Hardy-Weinberg equilibrium and its members continue called genetic drift).
to mate randomly generation after generation, allele and
genotype frequencies will remain constant. The system oper- By moving alleles into or out of populations,
ates somewhat like a deck of cards: No matter how many gene flow can alter allele frequencies.
times the deck is reshuffled to deal out new hands, the deck
itself remains the same. Aces do not grow more numerous Animation: Causes of Evolutionary Change
than jacks. And the repeated shuffling of a population’s
gene pool over the generations cannot, in itself, change the This disorder occurs in about one out of every 10,000 babies
frequency of one allele relative to another. born in the United States. Left untreated, PKU results in mental
disability and other problems. (As described in Concept 14.4,
Conditions for Hardy-Weinberg Equilibrium newborns are now routinely tested for PKU, and symptoms can
be largely avoided with a diet very low in phenylalanine.)
The Hardy-Weinberg approach describes a population that
is not evolving. This can occur if a population meets all five To apply the Hardy-Weinberg equation, we must assume
of the conditions for Hardy-Weinberg equilibrium listed in that no new PKU mutations are being introduced into the
Table 23.1. But in nature, the allele and genotype frequencies population (condition 1) and that people neither choose their
of a population often do change over time. Such changes can mates on the basis of whether or not they carry this gene nor
occur when at least one of the conditions for Hardy-Weinberg generally mate with close relatives (condition 2). We must
equilibrium is not met. also ignore any effects of differential survival and reproductive
success among PKU genotypes (condition 3) and assume that
Although departure from the conditions in Table 23.1 is there are no effects of genetic drift (condition 4) or of gene flow
common—resulting in evolutionary change—it is also com- from other populations into the United States (condition 5).
mon for natural populations to be in Hardy-Weinberg equilib- These assumptions are reasonable: The mutation rate for the
rium for specific genes. One way this can happen is if selection PKU gene is low, inbreeding and other forms of nonrandom
alters allele frequencies at some loci but not others. In addition, mating are not common in the United States, selection occurs
some populations evolve so slowly that the changes in their only against the rare homozygotes (and then only if dietary
allele and genotype frequencies are difficult to distinguish restrictions are not followed), the U.S. population is very large,
from those predicted for a non-evolving population. and populations outside the country have PKU allele frequen-
cies similar to those seen in the United States.
Applying the Hardy-Weinberg Equation
If all these assumptions hold, then the frequency of indi-
The Hardy-Weinberg equation is often used as an initial test viduals in the population born with PKU will correspond to
of whether evolution is occurring in a population (Concept q 2 in the Hardy-Weinberg equation (q 2 = frequency of homo-
Check 23.2, question 3 is an example). The equation also has zygotes). Because the allele is recessive, we must estimate the
medical applications, such as estimating the percentage of number of heterozygotes rather than counting them directly
a population carrying the allele for an inherited disease. as we did with the pink flowers. Recall that there is one PKU
For example, consider phenylketonuria (PKU), a metabolic occurrence per 10,000 births, which indicates that q 2 = 0.0001.
disorder that results from homozygosity for a recessive allele. Thus, the frequency (q) of the recessive allele for PKU is

490 Unit four  Mechanisms of Evolution q = 10.0001 = 0.01

and the frequency of the dominant allele is

p = 1 - q = 1 - 0.01 = 0.99

The frequency of carriers, heterozygous people who do not Concept  23.3
have PKU but may pass the PKU allele to offspring, is
Natural selection, genetic drift,
2pq = 2 * 0.99 * 0.01 = 0.0198 and gene flow can alter allele
(approximately 2% of the U.S. population) frequencies in a population

Remember, the assumption of Hardy-Weinberg equilib- Note again the five conditions required for a population
rium yields an approximation; the real number of carriers to be in Hardy-Weinberg equilibrium (see Table 23.1). A
may differ. Still, our calculations suggest that harmful reces- deviation from any of these conditions is a potential cause
sive alleles at this and other loci can be concealed in a popula- of evolution. New mutations (violation of condition 1) can
tion because they are carried by healthy heterozygotes. The alter allele frequencies, but because mutations are rare, the
Scientific Skills Exercise provides another opportunity for change from one generation to the next is likely to be very
you to apply the Hardy-Weinberg equation to allele data. small. Nonrandom mating (violation of condition 2) can
affect the frequencies of homozygous and heterozygous
Concept Check 23.2 genotypes but by itself has no effect on allele frequencies in
the gene pool. (Allele frequencies can change if individuals
1. A population has 700 individuals, 85 of genotype AA, 320 with certain inherited traits are more likely than other indi-
of genotype Aa, and 295 of genotype aa. What are the viduals to obtain mates. However, such a situation not only
frequencies of alleles A and a? causes a deviation from random mating, but also violates
condition 3, no natural selection.)
1. The frequency of allele a is 0.45 for a population in
Hardy-Weinberg equilibrium. What are the expected For the rest of this section we will focus on the three mech-
frequencies of genotypes AA, Aa, and aa? anisms that alter allele frequencies directly and cause most
evolutionary change: natural selection, genetic drift, and
1. WHAT IF? A locus that affects susceptibility to a degener- gene flow (violations of conditions 3–5).
ative brain disease has two alleles, V and v. In a population,
16 people have genotype V V, 92 have genotype V v, and
12 have genotype v v. Is this population ­evolving? Explain.
For suggested answers, see Appendix A.

Scientific Skills Exercise

Using the Hardy-Weinberg Equation 2. Next, use the Hardy-Weinberg
to Interpret Data and Make Predictions equation ( p2 + 2pq + q2 = 1) to
calculate the day 7 expected
Is Evolution Occurring in a Soybean Population?  One way frequencies of genotypes C GC G,
to test whether evolution is occurring in a population is to compare C GCY, and CYCY for a population
in Hardy-Weinberg equilibrium.
the observed genotype frequencies at a locus with those expected
3. Calculate the observed frequen-
for a non-evolving population based on the Hardy-Weinberg equa- cies of genotypes C GC G, C GCY,
and CYCY at day 7. Compare
tion. In this exercise, you’ll test whether a soybean population is these frequencies to the expected frequencies calculated in ques-
evolving at a locus with two alleles, C G and CY, that affect chloro- tion 2. Is the seedling population in Hardy-Weinberg equilibrium at
phyll production and hence leaf color. day 7, or is evolution occurring? Explain your reasoning and iden-
tify which genotypes, if any, appear to be selected for or against.
How the Experiment Was Done  Students planted soybean
seeds and then counted the number of seedlings of each genotype 4. Calculate the observed frequencies of genotypes C GC G, C GCY, and
CYCY at day 21. Compare these frequencies to the expected frequen-
at day 7 and again at day 21. Seedlings of each genotype could be cies calculated in question 2 and to the observed frequencies at
day 7. Is the seedling population in Hardy-Weinberg equilibrium at
distinguished visually because the C G and CY alleles show incomplete day 21, or is evolution occurring? Explain your reasoning and iden-
dominance: C GC G seedlings have green leaves, C GCY seedlings have tify which genotypes, if any, appear to be selected for or against.
green-yellow leaves, and CYCY seedlings have yellow leaves.
5. Homozygous CYCY individuals cannot produce chlorophyll. The
Data from the Experiment a­ bility to photosynthesize becomes more critical as seedlings age
and begin to exhaust the supply of food that was stored in the seed
Number of Seedlings from which they emerged. Develop a hypothesis that explains the
data for days 7 and 21. Based on this hypothesis, predict how the
Time Green Green-yellow Yellow Total frequencies of the C G and CY alleles will change beyond day 21.
(days) (C GC G) (C GCY) (CYCY) 216
173 Instructors: A version of this Scientific Skills Exercise can be
 7 49 111 56 assigned in MasteringBiology.

21 47 106 20

Interpret the Data

1. Use the observed genotype frequencies from the day 7 data to
calculate the frequencies of the C G allele ( p) and the CY allele (q).

chapter 23  The Evolution of Populations 491

Natural Selection Genetic Drift

The concept of natural selection is based on differential suc- If you flip a coin 1,000 times, a result of 700 heads and 300 tails
cess in survival and reproduction: Individuals in a popula- might make you suspicious about that coin. But if you flip a
tion exhibit variations in their heritable traits, and those coin only 10 times, an outcome of 7 heads and 3 tails would
with traits that are better suited to their environment tend to not be surprising. The smaller the number of coin flips, the
produce more offspring than those with traits that are not as more likely it is that chance alone will cause a deviation from
well suited. the predicted result. (In this case, the prediction is an equal
number of heads and tails.) Chance events can also cause
In genetic terms, selection results in alleles being passed to allele frequencies to fluctuate unpredictably from one genera-
the next generation in proportions that differ from those in tion to the next, especially in small populations—a process
the present generation. For example, the fruit fly D. melanogas- called genetic drift.
ter has an allele that confers resistance to several insecticides,
including DDT. This allele has a frequency of 0% in laboratory Figure 23.9 models how genetic drift might affect a small
strains of D. melanogaster established from flies collected in the population of our wildflowers. In this example, drift leads to
wild in the early 1930s, prior to DDT use. However, in strains the loss of an allele from the gene pool, but it is a matter of
established from flies collected after 1960 (following 20 or chance that the CW allele is lost and not the CR allele. Such
more years of DDT use), the allele frequency is 37%. We can unpredictable changes in allele frequencies can be caused
infer that this allele either arose by mutation between 1930 by chance events associated with survival and reproduction.
and 1960 or was present in 1930, but very rare. In any case, Perhaps a large animal such as a moose stepped on the three
the rise in frequency of this allele most likely occurred because CWCW individuals in generation 2, killing them and increas-
DDT is a powerful poison that is a strong selective force in ing the chance that only the CR allele would be passed to the
exposed fly populations. next generation. Allele frequencies can also be affected by
chance events that occur during fertilization. For example,
As the D. melanogaster example suggests, an allele that suppose two individuals of genotype CRCW had a small num-
confers resistance to an insecticide will increase in frequency ber of offspring. By chance alone, every egg and sperm pair
in a population exposed to that insecticide. Such changes are that generated offspring could happen to have carried the
not coincidental. By consistently favoring some alleles over CR allele and not the CW allele.
others, natural selection can cause adaptive evolution, a
process in which traits that enhance survival or reproduction Certain circumstances can result in genetic drift having
tend to increase in frequency over time. We’ll explore this a significant impact on a population. Two examples are the
process in more detail later in this chapter. founder effect and the bottleneck effect.

Figure 23.9  Genetic drift. This small wildflower population has a stable size of ten VISUAL SKILLS Based on this diagram, summarize
plants. Suppose that by chance only five plants of generation 1 (those highlighted in yellow) how the frequency of the CW allele changes over time.
produce fertile offspring. (This could occur, for example, if only those plants happened to
grow in a location that provided enough nutrients to support the production of offspring.)
Again by chance, only two plants of generation 2 leave fertile offspring.

Highlighted plants
leave offspring.

CRCR CRCW CRCR
CWCW CRCR
CRCR CRCW Only 5 of CRCR Only 2 of
CRCR 10 plants 10 plants CRCR
leave leave CRCR
CRCR offspring. offspring.
CRCW
CRCR
CWCW
CRCR
CWCW CWCW CRCR CRCR
CRCW
CRCR
CRCW
CRCW CRCW CRCR

CRCR CRCR

CRCW CRCR

Generation 1 Generation 2 Generation 3
p (frequency of C R)= 0.7 p = 0.5 p = 1.0
q (frequency of C W)= 0.3 q = 0.5 q = 0.0

492 Unit four  Mechanisms of Evolution

The Founder Effect recovers in size, it may have low levels of genetic variation
for a long period of time—a legacy of the genetic drift that
When a few individuals become isolated from a larger popula- occurred when the population was small.
tion, this smaller group may establish a new population whose
gene pool differs from the source population; this is called Human actions sometimes create severe bottlenecks for
the founder effect. The founder effect might occur, for other species, as the following example shows.
example, when a few members of a population are blown by
a storm to a new island. Genetic drift, in which chance events Case Study: Impact of Genetic Drift
alter allele frequencies, can occur in such a case because the on the Greater Prairie Chicken
storm indiscriminately transports some individuals (and their
alleles), but not others, from the source population. Millions of greater prairie chickens (Tympanuchus cupido)
once lived on the prairies of Illinois. As these prairies were
The founder effect probably accounts for the relatively converted to farmland and other uses during the 19th
high frequency of certain inherited disorders among isolated and 20th centuries, the number of greater prairie chickens
human populations. For example, in 1814, 15 British colo- plummeted (Figure 23.11a). By 1993 fewer than 50 birds
nists founded a settlement on Tristan da Cunha, a group of remained. These few surviving birds had low levels of genetic
small islands in the Atlantic Ocean midway between Africa variation, and less than 50% of their eggs hatched, compared
and South America. Apparently, one of the colonists carried a with much higher hatching rates of the larger populations in
recessive allele for retinitis pigmentosa, a progressive form of Kansas and Nebraska (Figure 23.11b).
blindness that afflicts homozygous individuals. Of the found-
ing colonists’ 240 descendants on the island in the late 1960s, Figure 23.11  Genetic drift and loss of genetic variation.
four had retinitis pigmentosa. The frequency of the allele that
causes this disease is ten times higher on Tristan da Cunha Pre-bottleneck Post-bottleneck
than in the populations from which the founders came. (Illinois, 1820) (Illinois, 1993)

The Bottleneck Effect Greater prairie chicken

A sudden change in the environment, such as a fire or flood, Range In 1993, with less than
may drastically reduce the size of a population. A severe drop of greater 1% of the grasslands
in population size can cause the bottleneck effect, so prairie remaining, the prairie
named because the population has passed through a “bottle- chicken chickens were found
neck” that reduces its size (Figure 23.10). By chance alone, Grasslands in which the in just two locations.
certain alleles may be overrepresented among the survivors, prairie chickens live once
others may be underrepresented, and some may be absent covered most of the state.
altogether. Ongoing genetic drift is likely to have substantial
effects on the gene pool until the population becomes large (a) The Illinois population of greater prairie chickens dropped from
enough that chance events have less impact. But even if a millions of birds in the 1800s to fewer than 50 birds in 1993.
population that has passed through a bottleneck ultimately
Location Population Number Percentage
Figure 23.10  The bottleneck effect. Shaking just a few marbles size of alleles of eggs
through the narrow neck of a bottle is analogous to a drastic reduction Illinois per locus hatched
in the size of a population. By chance, blue marbles are overrepresented 1930–1960s
in the surviving population and gold marbles are absent. 1993

1,000–25,000 5.2 93
<50 3.7 <50

Kansas, 1998 750,000 5.8 99
(no bottleneck)

Nebraska, 1998 75,000– 5.8 96
(no bottleneck) 200,000

Original Bottlenecking Surviving (b) In the small Illinois population, genetic drift led to decreases in the
population event population number of alleles per locus and the percentage of eggs hatched.

chapter 23  The Evolution of Populations 493

These data suggest that genetic drift during the bottleneck be lost or become fixed (reach a frequency of 100%) by
may have led to a loss of genetic variation and an increase in chance through genetic drift. In very small populations,
the frequency of harmful alleles. To investigate this hypothe- genetic drift can also cause alleles that are slightly harm-
sis, researchers extracted DNA from 15 museum specimens of ful to become fixed. When this occurs, the population’s
Illinois greater prairie chickens. Of the 15 birds, 10 had been survival can be threatened (as in greater prairie chickens).
collected in the 1930s, when there were 25,000 greater prairie
chickens in Illinois, and 5 had been collected in the 1960s, Gene Flow
when there were 1,000 greater prairie chickens in Illinois. By
studying the DNA of these specimens, the researchers were Natural selection and genetic drift are not the only phenom-
able to obtain a minimum, baseline estimate of how much ena affecting allele frequencies. Allele frequencies can also
genetic variation was present in the Illinois population before change by gene flow, the transfer of alleles into or out of a
the population shrank to extremely low numbers. This base- population due to the movement of fertile individuals or their
line estimate is a key piece of information that is not usually gametes. For example, suppose that near our original hypo-
available in cases of population bottlenecks. thetical wildflower population there is another population
consisting primarily of white-flowered individuals (CWCW).
The researchers surveyed six loci and found that the 1993 Insects carrying pollen from these plants may fly to and pol-
population had fewer alleles per locus than the pre-bottleneck linate plants in our original population. The introduced CW
Illinois or the current Kansas and Nebraska populations (see alleles would modify our original population’s allele frequen-
Figure 23.11b). Thus, as predicted, drift had reduced the genetic cies in the next generation. Because alleles are transferred
variation of the small 1993 population. Drift may also have between populations, gene flow tends to reduce the genetic
increased the frequency of harmful alleles, leading to the low differences between populations. In fact, if it is extensive
egg-hatching rate. To counteract these negative effects, 271 enough, gene flow can result in two populations combining
birds from neighboring states were added to the Illinois popu- into a single population with a common gene pool.
lation over four years. This strategy succeeded: New alleles
entered the population, and the egg-hatching rate improved to Alleles transferred by gene flow can also affect how well
over 90%. Overall, studies on the Illinois greater prairie chicken populations are adapted to local environmental condi-
illustrate the powerful effects of genetic drift in small popula- tions. For instance, mainland and island populations of the
tions and provide hope that in at least some populations, these Lake Erie water snake (Nerodia sipedon) differ in their color
effects can be reversed. patterns: Nearly all snakes from the Ohio or Ontario main-
lands are strongly banded, whereas the majority of snakes
Effects of Genetic Drift: A Summary from islands are unbanded or intermediate (Figure 23.12).
Banding coloration is an inherited trait, determined by a few
The examples we’ve described highlight four key points: loci (with alleles that encode bands being dominant to alleles
that encode the absence of bands). On islands, water snakes
1. Genetic drift is significant in small populations. live along rocky shorelines, while on the mainland, they live
Chance events can cause an allele to be disproportion- in marshes. Snakes without bands are more well camouflaged
ately over- or underrepresented in the next generation. in island habitats than are snakes with bands. Hence, on
Although chance events occur in populations of all sizes, islands, snakes without bands survive at higher rates than do
they tend to alter allele frequencies substantially only in snakes with bands.
small populations.
These data indicate that snakes without bands are favored
2. Genetic drift can cause allele frequencies to change by natural selection in island populations. Thus, we might
at random. Because of genetic drift, an allele may increase expect that all snakes on islands would lack bands. Why
in frequency one year, then decrease the next; the change is this not the case? The answer lies in gene flow from the
from year to year is not predictable. Thus, unlike natural mainland. In any given year, 3 to10 snakes from the main-
selection, which in a given environment consistently favors land swim to the islands and join the populations there. As
some alleles over others, genetic drift causes allele frequen- a result, each year such migrants transfer alleles for banded
cies to change at random over time. coloration from the mainland (where nearly all snakes have
bands) to the islands. This ongoing gene flow has prevented
3. Genetic drift can lead to a loss of genetic variation selection from removing all of the alleles for banded color-
within populations. By causing allele frequencies to ation from island populations—thereby preventing island
fluctuate randomly over time, genetic drift can eliminate populations from adapting fully to local conditions.
alleles from a population. Because evolution depends on
genetic variation, such losses can influence how effectively Gene flow can also transfer alleles that improve the abil-
a population can adapt to a change in the environment. ity of populations to adapt to local conditions. For example,
gene flow has resulted in the worldwide spread of several
4. Genetic drift can cause harmful alleles to become insecticide resistance alleles in the mosquito Culex pipiens,
fixed. Alleles that are neither harmful nor beneficial can

494 Unit four  Mechanisms of Evolution

Figure 23.12  Gene flow and local adaptation in the Lake members of populations that previously had very little
Erie water snake (Nerodia sipedon). Researchers assigned letters contact, leading to an exchange of alleles and fewer genetic
to variations in coloration in N. sipedon populations. Color pattern A differences between those populations.
is strong banding, patterns B and C are intermediate banding, and
pattern D is no banding. Banding is advantageous for camouflage BioFlix® Animation: Natural Selection, Genetic Drift,
in mainland environments, whereas having no bands is advantageous and Gene Flow
in island environments. However, gene flow from the mainland causes
banding to persist in island populations. Concept Check 23.3

Detroit ONTARIO Unbanded 1. In what sense is natural selection more “predictable”
LAKE ERIE N. sipedon than genetic drift?
(Pattern D)
OHIO Cleveland 2. Distinguish genetic drift from gene flow in terms of
Pelee (a) how they occur and (b) their implications for future
BASS Island genetic variation in a population.
ISLANDS Middle
Island 3. WHAT IF? Suppose two plant populations exchange
pollen and seeds. In one population, individuals of geno-
LAKE ERIE type AA are most common (9,000 AA, 900 Aa, 100 aa),
while the opposite is true in the other population
OHIO Kelleys (100 AA, 900 Aa, 9,000 aa). If neither allele has a selective
Island advantage, what will happen over time to the allele
Banded N. sipedon and genotype frequencies of these populations?
(Pattern C) 5 km For suggested answers, see Appendix A.

Percentage of individuals 100 Concept  23.4
80
Natural selection is the only
60 mechanism that consistently
40 causes adaptive evolution

20 Evolution by natural selection is a blend of chance and
“sorting”: chance in the creation of new genetic variations (as in
0 A BCD A BCD mutation) and sorting as natural selection favors some alleles
A BCD Islands Ontario mainland over others. Because of this favoring process, the outcome of
Ohio mainland natural selection is not random. Instead, natural selection con-
sistently increases the frequencies of alleles that provide repro-
Banding patterns in snake populations ductive advantage, thus leading to adaptive evolution.

WHAT IF? Suppose a severe weather event caused island populations to Natural Selection: A Closer Look
decrease in size but did not affect the size of mainland populations. Predict
To see how natural selection can cause adaptive evolution,
how gene flow from the mainland would affect color patterns in island we’ll begin with the concept of relative fitness and the differ-
ent ways that selection acts on an organism’s phenotype.
populations. Explain.
Relative Fitness
a vector of West Nile virus and other diseases. Each of these
alleles has a unique genetic signature that allowed research- The phrases “struggle for existence” and “survival of the
ers to document that it arose by mutation in only one or fittest” are commonly used to describe natural selection,
a few geographic locations. In their population of origin, but these expressions are misleading if always taken to
these alleles increased because they provided insecticide mean direct competitive contests among individuals. There
resistance. These alleles were then transferred to new popu- are animal species in which individuals, usually the males,
lations, where again, their frequencies increased as a result lock horns or otherwise do combat to determine mating
of natural selection. privilege. But reproductive success is generally more subtle
and depends on many factors besides outright battle. For
Finally, gene flow has become an increasingly impor- example, a barnacle that is more efficient at collecting food
tant agent of evolutionary change in human populations. than its neighbors may have greater stores of energy and
Humans today move much more freely about the world than hence be able to produce a larger number of eggs. A moth
in the past. As a result, mating is more common between may have more offspring than other moths in the same
population because its body colors more effectively conceal

chapter 23  The Evolution of Populations 495

it from predators, improving its chance of surviving long character in one direction or the other (Figure 23.13a).
enough to produce more offspring. These examples illus-
trate how in a given environment, certain traits can lead to Directional selection is common when a population’s environ-
greater relative fitness: the contribution an individual
makes to the gene pool of the next generation relative to the ment changes or when members of a population migrate to
contributions of other individuals.
a new (and different) habitat. For instance, an increase in the
Although we often refer to the relative fitness of a geno-
type, remember that the entity that is subjected to natural relative abundance of large seeds over small seeds led to an
selection is the whole organism, not the underlying geno-
type. Thus, selection acts more directly on the phenotype increase in beak depth in a population of Galápagos finches
than on the genotype; it acts on the genotype indirectly,
via how the genotype affects the phenotype. (see Figure 23.2).

Directional, Disruptive, and Stabilizing Selection Disruptive selection (Figure 23.13b) occurs when con-

Natural selection can alter the frequency distribution of heri- ditions favor individuals at both extremes of a phenotypic
table traits in three ways, depending on which phenotypes in
a population are favored: through directional selection, dis- range over individuals with intermediate phenotypes. One
ruptive selection, and stabilizing selection.
example is a population of black-bellied seedcracker finches
Directional selection occurs when conditions favor indi-
viduals exhibiting one extreme of a phenotypic range, thereby in Cameroon whose members display two distinctly differ-
shifting a population’s frequency curve for the phenotypic
ent beak sizes. Small-billed birds feed mainly on soft seeds,
Figure 23.13  Modes of selection.
These cases describe three ways in which a whereas large-billed birds specialize in cracking hard seeds. It
hypothetical deer mouse population with
heritable variation in fur coloration might appears that birds with intermediate-sized bills are relatively
evolve. The graphs show how the frequencies
of individuals with different fur colors change inefficient at cracking both types of seeds and thus have lower
over time. The large white arrows symbolize
selective pressures against certain phenotypes. relative fitness.

MAKE CONNECTIONS Review Figure 22.13. Stabilizing selection (Figure 23.13c) acts against both
Which mode of selection has occurred in soapberry
bug populations that feed on the introduced extreme phenotypes and favors intermediate variants. This
goldenrain tree? Explain.
mode of selection reduces variation and tends to maintain

the status quo for a particular phenotypic character. For

example, the birth weights of most

human babies lie in the range of 3–4 kg

Frequency of individuals Original (6.6–8.8 pounds); babies who are either
population much smaller or much larger suffer

higher rates of mortality.

Regardless of the mode of selection,

however, the basic mechanism remains

the same. Selection favors individuals

whose heritable phenotypic traits pro-

vide higher reproductive success than

do the traits of other individuals.

Original Evolved Phenotypes (fur color)

population population

(a) Directional selection shifts the overall (b) Disruptive selection favors variants (c) Stabilizing selection removes
makeup of the population by favoring at both ends of the distribution. These extreme variants from the population
variants that are at one extreme of the mice have colonized a patchy habitat and preserves intermediate types. If
distribution. In this case, lighter mice made up of light and dark rocks, with the environment consists of rocks of
are selected against because they live the result that mice of an intermediate an intermediate color, both light and
among dark rocks, making it harder for color are selected against. dark mice will be selected against.
them to hide from predators.

496 Unit four  Mechanisms of Evolution

The Key Role of Natural Selection increase, but it also can cause the frequency of such an allele
in Adaptive Evolution to decrease. Similarly, gene flow may introduce alleles that
are advantageous or ones that are disadvantageous. Natural
The adaptations of organisms include many striking exam- selection is the only evolutionary mechanism that consis-
ples. Certain octopuses, for example, have the ability to tently leads to adaptive evolution.
change color rapidly, enabling them to blend into different
backgrounds. Another example is the remarkable jaws of HHMI Video: Got Lactase? The Co-evolution
snakes (Figure 23.14), which allow them to swallow prey of Genes and Culture
much larger than their own head (a feat analogous to a per-
son swallowing a whole watermelon). Other adaptations, Sexual Selection
such as a version of an enzyme that shows improved function
in cold environments, may be less visually dramatic but just Charles Darwin was the first to explore the implications of
as important for survival and reproduction. sexual selection, a process in which individuals with certain
inherited characteristics are more likely than other individu-
Such adaptations can arise gradually over time as natural als of the same sex to obtain mates. Sexual selection can result
selection increases the frequencies of alleles that enhance sur- in sexual dimorphism, a difference in secondary sexual
vival or reproduction. As the proportion of individuals that characteristics between males and females of the same species
have favorable traits increases, the degree to which a species is (Figure 23.15). These distinctions include differences in size,
well suited for life in its environment improves; that is, adap- color, ornamentation, and behavior.
tive evolution occurs. However, the physical and biological
components of an organism’s environment may change over How does sexual selection operate? There are several ways.
time. As a result, what constitutes a “good match” between In intrasexual selection, meaning selection within the
an organism and its environment can be a moving target, same sex, individuals of one sex compete directly for mates
making adaptive evolution a continuous, dynamic process. of the opposite sex. In many species, intrasexual selection
Environmental conditions can also differ from place to place, occurs among males. For example, a single male may patrol a
causing different alleles to be favored in different locations. group of females and prevent other males from mating with
When this occurs, natural selection can cause the populations them. The patrolling male may defend his status by defeating
of a species to differ genetically from one another. smaller, weaker, or less fierce males in combat. More often,
this male is the psychological victor in ritualized displays that
And what about genetic drift and gene flow? Both can, in discourage would-be competitors but do not risk injury that
fact, increase the frequencies of alleles that enhance survival would reduce his own fitness (see Figure 51.16). Intrasexual
or reproduction, but neither does so consistently. Genetic selection also occurs among females in a variety of species,
drift can cause the frequency of a slightly beneficial allele to including ring-tailed lemurs and broadnosed pipefish.

Figure 23.14  Movable jaw bones in snakes. In intersexual selection, also called mate choice, individ-
uals of one sex (usually the females) are choosy in selecting
The bones of the upper their mates from the other sex. In many cases, the female’s
jaw that are shown in choice depends on the showiness of the male’s appearance
green are movable.
Figure 23.15  Sexual dimorphism and sexual selection. A
Ligament peacock (left) and a peahen (right) show extreme sexual dimorphism.
There is intrasexual selection between competing males, followed by
intersexual selection when the females choose among the showiest males.

The skull bones of 497
most terrestrial
vertebrates are
relatively rigidly
attached to one
another, limiting jaw
movement. In contrast,
most snakes have
movable bones in their
upper jaw, allowing
them to swallow food
much larger than
their head.
chapter 23  The Evolution of Populations

or behavior (see Figure 23.15). What intrigued Darwin Figure 23.16
about mate choice is that male showiness may not seem
adaptive in any other way and may in fact pose some risk. Inquiry  Do females select mates based
For example, bright plumage may make male birds more on traits indicative of “good genes”?
visible to predators. But if such characteristics help a male
gain a mate, and if this benefit outweighs the risk from pre- Experiment  Female gray tree frogs (Hyla versicolor) prefer to
dation, then both the bright plumage and the female pref- mate with males that give long mating calls. Allison Welch and col-
erence for it will be reinforced because they enhance overall leagues, at the University of Missouri, tested whether the genetic
reproductive success. makeup of long-calling (LC) males is superior to that of short-calling
(SC) males. The researchers fertilized half the eggs of each female
How do female preferences for certain male characteris- with sperm from an LC male and fertilized the remaining eggs with
tics evolve in the first place? One hypothesis is that females sperm from an SC male. In two separate experiments (one in 1995,
prefer male traits that are correlated with “good genes.” If the other in 1996), the resulting half-sibling offspring were raised
the trait preferred by females is indicative of a male’s overall in a common environment and their survival and growth were
genetic quality, both the male trait and female preference for monitored.
it should increase in frequency. Figure 23.16 describes one
experiment testing this hypothesis in gray tree frogs. Recording of SC Recording of LC
male’s call male’s call
Other researchers have shown that in several bird species,
the traits preferred by females are related to overall male health. Female gray
Here, too, female preference appears to be based on traits that tree frog
reflect “good genes,” in this case, alleles indicative of a robust
immune system. SC male gray LC male gray
tree frog tree frog
Balancing Selection LC sperm
SC sperm × Eggs ×
As we’ve seen, genetic variation is often found at loci affected
by selection. What prevents natural selection from reducing Offspring of Offspring of
the variation at those loci by culling all unfavorable alleles? SC father LC father
As mentioned earlier, in diploid organisms, many unfavorable
recessive alleles persist because they are hidden from selection Survival and growth of these half-sibling offspring compared
when in heterozygous individuals. In addition, selection itself
may preserve variation at some loci, thus maintaining two or Results 1995 1996
more phenotypic forms in a population. Known as balancing LC better NSD
selection, this type of selection includes frequency-dependent Offspring NSD LC better
selection and heterozygote advantage. Performance LC better (shorter) LC better (shorter)
Larval survival
Frequency-Dependent Selection Larval growth
Time to
In frequency-dependent selection, the fitness of a metamorphosis
phenotype depends on how common it is in the popula-
tion. Consider the scale-eating fish (Perissodus microlepis) of NSD = no significant difference; LC better = offspring of LC males superior to offspring
Lake Tanganyika, in Africa. These fish attack other fish from of SC males.
behind, darting in to remove a few scales from the flank of
their prey. Of interest here is a peculiar feature of the scale- Conclusion  Because offspring fathered by an LC male outperformed
eating fish: Some are “left-mouthed” and some are “right- their half-siblings fathered by an SC male, the team concluded that
mouthed.” This trait is determined by two alleles and simple the duration of a male’s mating call is indicative of the male’s overall
Mendelian inheritance. Hence, all individuals in a population genetic quality. This result supports the hypothesis that female mate
are either left-mouthed or right-mouthed, and the frequen- choice can be based on a trait that indicates whether the male has
cies of these two phenotypes must add up to 100%. “good genes.”

Because their mouth twists to the left, left-mouthed fish Data from  A. M. Welch et al., Call duration as an indicator of genetic quality in male
always attack their prey’s right flank (Figure 23.17). (To see gray tree frogs, Science 280:1928–1930 (1998).
why, twist your lower jaw and lips to the left and imagine
trying to take a bite from the left side of a fish, approaching Inquiry in Action  Read and analyze the original paper in Inquiry in
it from behind.) Similarly, right-mouthed fish always attack Action: Interpreting Scientific Papers.
from the left. Prey species guard against attack from whatever
phenotype of scale-eating fish is most common in the lake. WHAT IF?  Why did the researchers split each female frog’s eggs into two
batches for fertilization by different males? Why didn’t they mate each female
498 Unit four  Mechanisms of Evolution with a single male frog?

Figure 23.17  Frequency-dependent selection. In a populationFrequency of sickle-cell disease. The red blood cells of people with sickle-cell
of the scale-eating fish Perissodus microlepis, the frequency of left-left-mouthed individualsdisease become distorted in shape, or sickled, under low-oxygen
mouthed individuals (red data points) rises and falls in a regular manner. conditions (see Figure 5.19), as occurs in the capillaries. These
The frequency of left-mouthed individuals among adults that reproduced sickled cells can clump together and block the flow of blood in
was also recorded in three sample years (green data points). the capillaries, damaging organs such as the kidney, heart, and
brain. Although some red blood cells become sickled in hetero-
”Left-mouthed” zygotes, not enough become sickled to cause sickle-cell disease.
P. microlepis
Heterozygotes for the sickle-cell allele are protected against
1.0 the most severe effects of malaria, a disease caused by a para-
”Right-mouthed” site that infects red blood cells (see Figure 28.16). One reason
P. microlepis for this partial protection is that the body destroys sickled red
blood cells rapidly, killing the parasites they harbor. Malaria
0.5 is a major killer in some tropical regions. In such regions,
selection favors heterozygotes over homozygous dominant
0 ‘83 ‘85 ‘87 ‘89 individuals, who are more vulnerable to the effects of malaria,
1981 Sample year and also over homozygous recessive individuals, who develop
sickle-cell disease. As described in Figure 23.18, these selec-
INTERPRET THE DATA For 1981, 1987, and 1990, compare the tive pressures have caused the frequency of the sickle-cell
frequency of left-mouthed individuals among breeding adults to the frequency allele to reach relatively high levels in areas where the malaria
parasite is common.
of left-mouthed individuals in the entire population. What do the data indicate
Why Natural Selection Cannot Fashion
about when natural selection favors left-mouthed individuals over right- Perfect Organisms

mouthed individuals (or vice versa)? Explain. Though natural selection leads to adaptation, nature abounds
with examples of organisms that are less than ideally suited
Thus, from year to year, selection favors whichever mouth for their lifestyles. There are several reasons why.
phenotype is least common. As a result, the frequency of left-
and right-mouthed fish oscillates over time, and balancing 1. Selection can act only on existing variations.
selection (due to frequency dependence) keeps the frequency Natural selection favors only the fittest phenotypes
of each phenotype close to 50%. among those currently in the population, which may not
be the ideal traits. New advantageous alleles do not arise
Heterozygote Advantage on demand.

If individuals who are heterozygous at a particular locus 2. Evolution is limited by historical constraints. Each
have greater fitness than do both kinds of homozygotes, they species has a legacy of descent with modification from
exhibit heterozygote advantage. In such a case, natural ancestral forms. Evolution does not scrap the ancestral anat-
selection tends to maintain two or more alleles at that locus. omy and build each new complex structure from scratch;
Note that heterozygote advantage is defined in terms of rather, evolution co-opts existing structures and adapts
genotype, not phenotype. Thus, whether heterozygote advan- them to new situations. We could imagine that if a ter-
tage represents stabilizing or directional selection depends restrial animal were to adapt to an environment in which
on the relationship between the genotype and the pheno- flight would be advantageous, it might be best just to grow
type. For example, if the phenotype of a heterozygote is an extra pair of limbs that would serve as wings. However,
intermediate to the phenotypes of both homozygotes, evolution does not work this way; instead, it operates on
heterozygote advantage is a form of stabilizing selection. the traits an organism already has. Thus, in birds and bats,
an existing pair of limbs took on new functions for flight
An example of heterozygote advantage occurs at the locus as these organisms evolved from nonflying ancestors.
in humans that codes for the β polypeptide subunit of hemo-
globin, the oxygen-carrying protein of red blood cells. In 3. Adaptations are often compromises. Each organism
homozygous individuals, a recessive allele at that locus causes must do many different things. A seal spends part of its
time on rocks; it could probably walk better if it had legs in-
stead of flippers, but then it would not swim nearly as well.
We humans owe much of our versatility and athleticism
to our prehensile hands and flexible limbs, but these also
make us prone to sprains, torn ligaments, and dislocations:
Structural reinforcement has been compromised for agility.

chapter 23  The Evolution of Populations 499

Figure 23.18  MAKE CONNECTIONS

The Sickle-Cell Allele

This child has sickle-cell disease, a genetic disorder that strikes individuals
that have two copies of the sickle-cell allele. This allele causes an
abnormality in the structure and function of hemoglobin, the oxygen-
carrying protein in red blood cells. Although sickle-cell disease is lethal if not
treated, in some regions the sickle-cell allele can reach frequencies as high as
15–20%. How can such a harmful allele be so common?

Events at the Molecular Level Consequences for Cells

• Due to a point mutation, the sickle-cell allele differs • The abnormal hemoglobin fibers distort the red blood
from the wild-type allele by a single nucleotide. (See Figure 17.26.) cell into a sickle shape under low-oxygen conditions,
such as those found in blood vessels returning to the heart.
• The resulting change in one amino acid leads to hydrophobic
interactions between the sickle-cell hemoglobin proteins under Fiber
low-oxygen conditions.

• As a result, the sickle-cell proteins bind to each other in chains that
together form a fiber.

Template strand

Sickle-cell allele
on chromosome
T C
C C T C T A G
G G A G A
A
G G A C T
C C T G
A
C T G T
G A C
A
G T
C

An adenine replaces a thymine in the Sickle-cell Low-oxygen
template strand of the sickle-cell allele, hemoglobin conditions
changing one codon in the mRNA
produced during transcription. This
change causes an amino-acid change in
sickle-cell hemoglobin: A valine replaces
a glutamic acid at one position. (See Figure 5.19.)

Wild-type Sickled red blood cell

allele T C
A G
C C T C T
C T G G A G A
G A
G G A
A C C T
T
C T G
A G A C
T
G
C

HHMI Video: The Making of Normal hemoglobin
the Fittest: Natural Selection in (does not aggregate into fibers)
Humans (Sickle-Cell Disease)
500 Unit four  Mechanisms of Evolution Normal red blood cell

Effects on Individual Organisms Infected mosquitoes
spread malaria when they
• The formation of sickled red blood cells causes homozygotes bite people. (See Figure
with two copies of the sickle-cell allele to have sickle-cell disease. 28.16.)

• Some sickling also occurs in heterozygotes, but not enough to Evolution in Populations
cause the disease; they have sickle-cell trait. (See Figure 14.17.)
• Homozygotes with two sickle-cell alleles are
The sickled blood cells of a strongly selected against because of mortality caused
homozygote block small blood by sickle-cell disease. In contrast, heterozygotes
vessels, causing great pain and experience few harmful effects from sickling yet are
damage to organs such as the more likely to survive malaria than are homozygotes.
heart, kidney, and brain.
• In regions where malaria is common, the net effect
of these opposing selective forces is heterozygote
advantage. This has caused evolutionary change in
populations—the products of which are the areas of
relatively high frequencies of the sickle-cell allele
shown in the map below.

Normal red blood cells Key
are flexible and are able Frequencies of
to flow freely through the sickle-cell allele
small blood vessels.
3.0 –6.0%
6.0 –9.0% Distribution of malaria
9.0 –12.0% caused by Plasmodium falciparum
12.0 –15.0% (a parasitic unicellular eukaryote)

>15.0%

MAKE CONNECTIONS In a region free of malaria, would individuals who are
heterozygous for the sickle-cell allele be selected for or selected against? Explain.

chapter 23  The Evolution of Populations 501

4. Chance, natural selection, and the environment than” basis. We can, in fact, see evidence for evolution in the
interact. Chance events can affect the subsequent evolu- many imperfections of the organisms it produces.
tionary history of populations. For instance, when a storm
blows insects or birds hundreds of kilometers over an ocean Concept Check 23.4
to an island, the wind does not necessarily transport those
individuals that are best suited to the new environment. 1. What is the relative fitness of a sterile mule? Explain.
Thus, not all alleles present in the founding population’s 2. Explain why natural selection is the only evolutionary
gene pool are better suited to the new environment than
the alleles that are “left behind.” In addition, the environ- mechanism that consistently leads to adaptive evolution
ment at a particular location may change unpredictably in a population.
from year to year, again limiting the extent to which 3. VISUAL SKILLS Consider a population in which hetero-
adaptive evolution results in organisms being well zygotes at a certain locus have an extreme phenotype
suited for current environmental conditions. (such as being larger than homozygotes) that confers
a selective advantage. Compare this description to the
With these four constraints, evolution does not tend to models of selection modes shown in Figure 23.13. Does
craft perfect organisms. Natural selection operates on a “better this situation represent directional, disruptive, or stabiliz-
ing selection? Explain your answer.

For suggested answers, see Appendix A.

23 Chapter Review Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz,
Practice Tests, and more in the Study Area.

Summary of Key Concepts Concept  23.3

Concept  23.1 Natural selection, genetic drift, and gene flow can
alter allele frequencies in a population (pp. 491–495)
Genetic variation makes evolution
possible (pp. 485–487) In natural selection, individuals that have certain inherited
traits tend to survive and reproduce at higher rates than other
Genetic variation refers to genetic differences VOCAB individuals because of those traits.
among individuals within a population. SELF-QUIZ
goo.gl/6u55ks In genetic drift, chance fluctuations in allele frequencies over
generations tend to reduce genetic variation.
The nucleotide differences that provide the basis
of genetic variation originate when mutation and gene duplica- Gene flow, the transfer of alleles between populations, tends
tion produce new alleles and new genes. New genetic variants are to reduce genetic differences between populations over time.

produced rapidly in organisms with short generation times. In ? Would two small, geographically isolated populations in very different
environments be likely to evolve in similar ways? Explain.
sexually reproducing organisms, most of the genetic differences
among individuals result from crossing over, the independent
assortment of chromosomes, and fertilization. Concept  23.4

? Typically, most of the nucleotide variability that occurs within a genetic Natural selection is the only mechanism that
locus does not affect the phenotype. Explain why. consistently causes adaptive evolution (pp. 495–502)

Concept  23.2 One organism has greater relative fitness than another organ-
ism if it leaves more fertile descendants. The modes of natural
The Hardy-Weinberg equation can be used to selection differ in their effect on phenotype:
test whether a population is evolving (pp. 487–491)
Original population Evolved population

A population, a localized group of organisms belonging to one Directional selection Disruptive selection Stabilizing selection
species, is united by its gene pool, the aggregate of all the alleles
in the population. Unlike genetic drift and gene flow, natural selection consistently
increases the frequencies of alleles that enhance survival and
For a population in Hardy-Weinberg equilibrium, the allele reproduction, thus improving the degree to which organisms are
and genotype frequencies will remain constant if the population well-suited for life in their environment.
is large, mating is random, mutation is negligible, there is no gene
flow, and there is no natural selection. For such a population, if Sexual selection can result in secondary sex characteristics that
p and q represent the frequencies of the only two possible alleles can give individuals advantages in mating.
at a particular locus, then p 2 is the frequency of one kind of
homozygote, q 2 is the frequency of the other kind of homozygote,
and 2pq is the frequency of the heterozygous genotype.

? Is it circular reasoning to calculate p and q from observed genotype
frequencies and then use those values of p and q to test if the

population is in Hardy-Weinberg equilibrium? Explain your answer.

502 Unit FOUR  Mechanisms of Evolution

Balancing selection occurs when natural selection maintains Sampling sites
two or more forms in a population. (1–8 represent 1 2 3 4 5 6 7 8 9 10 11
pairs of sites)
There are constraints to evolution: Natural selection can act only
on available variation; structures result from modified ancestral Allele
anatomy; adaptations are often compromises; and chance, natu- frequencies
ral selection, and the environment interact.

? How might secondary sex characteristics in males differ from those in
females in a species in which females compete for mates?
lap94 alleles Other lap alleles

Test Your Understanding Data from R. K. Koehn and T. J. Hilbish, The adaptive importance of genetic variation,
American Scientist 75:134–141 (1987).

Level 1: Knowledge/Comprehension Salinity increases toward the open ocean

1. Natural selection changes allele frequencies
because some _________ survive and reproduce
better than others. 2 3 4 5 67 8
(A) alleles (C) species PRACTICE
(B) loci (D) individuals TEST Long Island
Sound
goo.gl/CUYGKD 1

2. No two people are genetically identical, except for identical 9
twins. The main source of genetic variation among humans is
(A) new mutations that occurred in the preceding generation. N 10 Atlantic
(B) genetic drift. 11 Ocean
(C) the reshuffling of alleles in sexual reproduction.
(D) environmental effects.

Level 2: Application/Analysis mechanisms that can alter allele frequency, construct a
hypothesis that explains the patterns you observe in the data
3. If the nucleotide variability of a locus equals 0%, what is the and that accounts for the following observations: (1) The lap94
gene variability and number of alleles at that locus? allele helps mussels maintain osmotic balance in water with a
(A) gene variability = 0%; number of alleles = 0 high salt concentration but is costly to use in less salty water;
(B) gene variability = 0%; number of alleles = 1 and (2) mussels produce larvae that can disperse long distances
(C) gene variability = 0%; number of alleles = 2 before they settle on rocks and grow into adults.
(D) gene variability 7 0%; number of alleles = 2
8. WRITE ABOUT A THEME: Organization Heterozygotes
4. There are 25 individuals in population 1, all with genotype AA, at the sickle-cell locus produce both normal and abnormal
and there are 40 individuals in population 2, all with genotype (sickle-cell) hemoglobin (see Concept 14.4). When hemoglobin
aa. Assume that these populations are located far from each molecules are packed into a heterozygote’s red blood cells,
other and that their environmental conditions are very similar. some cells receive relatively large quantities of abnormal
Based on the information given here, the observed genetic hemoglobin, making these cells prone to sickling. In a short
variation most likely resulted from essay (approximately 100–150 words), explain how these
(A) genetic drift. (C) nonrandom mating. molecular and cellular events lead to emergent properties at the
(B) gene flow. (D) directional selection. individual and population levels of biological organization.

5. A fruit fly population has a gene with two alleles, A1 and A2. 9. SYNTHESIZE YOUR KNOWLEDGE 
Tests show that 70% of the gametes produced in the population
contain the A1 allele. If the population is in Hardy-Weinberg This kettle lake
equilibrium, what proportion of the flies carry both A1 and A2? formed 14,000 years
(A) 0.7 (B) 0.49 (C) 0.42 (D) 0.21 ago when a glacier
that covered the
Level 3: Synthesis/Evaluation surrounding area
melted. Initially
6. EVOLUTION CONNECTION  Using at least two examples, devoid of animal
explain how the process of evolution is revealed by the life, over time the
imperfections of living organisms. lake was colonized
by invertebrates
7. SCIENTIFIC INQUIRY • INTERPRET THE DATA Researchers and other animals.
studied genetic variation in the marine mussel Mytilus edulis Hypothesize how
around Long Island, New York. They measured the frequency of mutation, natural
a particular allele (lap 94) for an enzyme involved in regulating the selection, genetic
mussel’s internal saltwater balance. The researchers presented drift, and gene flow
their data as a series of pie charts linked to sampling sites within may have affected
Long Island Sound, where the salinity is highly variable, and populations that
along the coast of the open ocean, where salinity is constant. colonized the lake.
(a) Create a data table for the 11 sampling sites by estimating
the frequency of lap 94 from the pie charts. (Hint: Think of each For selected answers, see Appendix A.
pie chart as a clock face to help you estimate the proportion
of the shaded area.) (b) Graph the frequencies for sites 1–8 to For additional practice questions, check out the Dynamic Study
show how the frequency of this allele changes with increasing Modules in MasteringBiology. You can use them to study on
salinity in Long Island Sound (from southwest to northeast). your smartphone, tablet, or computer anytime, anywhere!
Evaluate how the data from sites 9–11 compare with the data
from the sites within the Sound. (c) Considering the various

chapter 23  The Evolution of Populations 503

The Origin of Species 24

Figure 24.1  How did this flightless bird come to live on the isolated Galápagos Islands?

Key Concepts That “Mystery of Mysteries”

24.1 The biological species concept When Darwin came to the Galápagos Islands, he noted that these volcanic islands
were teeming with plants and animals found nowhere else in the world (Figure 24.1).
emphasizes reproductive isolation Later he realized that these species had formed relatively recently. He wrote in his
diary, “Both in space and time, we seem to be brought somewhat near to that great
24.2 Speciation can take place with or fact—that mystery of mysteries—the first appearance of new beings on this Earth.”

without geographic separation The “mystery of mysteries” that captivated Darwin is speciation, the process
by which one species splits into two or more species. Speciation fascinated Darwin
24.3 Hybrid zones reveal factors that (and many biologists since) because it has produced the tremendous diversity of life,
repeatedly yielding new species that differ from existing ones. Later, Darwin real-
cause reproductive isolation ized that speciation also helps to explain the many features that organisms share
(the unity of life): When one species splits into two, the species that result share
24.4 Speciation can occur rapidly many characteristics because they are descended from this common ancestor. At
the DNA sequence level, for example, such similarities indicate that the flightless
or slowly and can result from cormorant (Phalacrocorax harrisi) in Figure 24.1 is closely related to flying cormo-
changes in few or many genes rants found in the Americas. This suggests that the flightless cormorant originated
from an ancestral cormorant species that flew from the mainland to the Galápagos.
Galápagos giant tortoise, another
species unique to the islands Speciation also forms a conceptual bridge between microevolution,
changes over time in allele frequencies in a population, and macroevolution,
the broad pattern of evolution above the species level. An example of

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504

macroevolutionary change is the origin of new groups of (a) Similarity between different species. The eastern meadowlark
organisms, such as mammals or flowering plants, through a (Sturnella magna, left) and the western meadowlark (Sturnella
series of speciation events. We examined microevolutionary neglecta, right) have similar body shapes and colorations.
mechanisms in Chapter 23, and we’ll turn to macroevolu- Nevertheless, they are distinct biological species because their
tion in Chapter 25. In this chapter, we’ll explore the “bridge” songs and other behaviors are different enough to prevent
between microevolution and macroevolution—the mecha- interbreeding should they meet in the wild.
nisms by which new species originate from existing ones. First,
let’s establish what we actually mean by a “species.”

HHMI Video: The Origin of Species:
The Beak of the Finch

Concept  24.1

The biological species concept
emphasizes reproductive isolation

The word species is Latin for “kind” or “appearance.” In daily
life, we commonly distinguish between various “kinds”
of organisms—dogs and cats, for instance—based on differ-
ences in their appearance. But are organisms truly divided
into the discrete units we call species? To answer this ques-
tion, biologists compare not only the morphology (body
form) of different groups of organisms but also less obvious
differences in physiology, biochemistry, and DNA sequences.
The results generally confirm that morphologically distinct
species are indeed discrete groups, differing in many ways
besides their body forms.

The Biological Species Concept (b) Diversity within a species. As diverse as we may be in appearance,
all humans belong to a single biological species (Homo sapiens),
The primary definition of species used in this textbook is the defined by our capacity to interbreed successfully.
biological species concept. According to this concept, a
species is a group of populations whose members have the Figure 24.2  The biological species concept is based
potential to interbreed in nature and produce viable, fertile on the potential to interbreed, not on physical similarity.
offspring—but do not produce viable, fertile offspring with
members of other such groups (Figure 24.2). Thus, the members Reproductive Isolation
of a biological species are united by being reproductively com-
patible, at least potentially. All human beings, for example, Because biological species are defined in terms of reproduc-
belong to the same species. A businesswoman in Manhattan tive compatibility, the formation of a new species hinges on
may be unlikely to meet a dairy farmer in Mongolia, but if the reproductive isolation—the existence of biological factors
two should happen to meet and mate, they could have viable (barriers) that impede members of two species from inter-
babies who develop into fertile adults. In contrast, humans and breeding and producing viable, fertile offspring. Such barriers
chimpanzees remain distinct biological species, even where block gene flow between the species and limit the formation
they live in the same region, because many factors keep them of hybrids, offspring that result from an interspecific mat-
from interbreeding and producing fertile offspring. ing. Although a single barrier may not prevent all gene flow,
a combination of several barriers can effectively isolate a
What holds the gene pool of a species together, causing species’ gene pool.
its members to resemble each other more than they resemble
members of other species? Recall the evolutionary mecha- Clearly, a fly cannot mate with a frog or a fern, but
nism of gene flow, the transfer of alleles between populations the reproductive barriers between more closely related
(see Concept 23.3). Typically, gene flow occurs between the species are not so obvious. As described in Figure 24.3,
different populations of a species. This ongoing exchange of
alleles tends to hold the populations together genetically.
But as we’ll explore in this chapter, a reduction or lack of gene
flow can play a key role in the formation of new species.

chapter 24  The Origin of Species 505

Figure 24.3  Exploring Reproductive Barriers

Prezygotic barriers impede mating or hinder fertilization if mating does occur

Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation

Individuals MATING
of ATTEMPT

different
species

Two species that occupy Species that breed during Courtship rituals that attract Mating is attempted, but
different habitats within the different times of the day, mates and other behaviors morphological differences
same area may encounter each different seasons, or different unique to a species are effective prevent its successful
other rarely, if at all, even years cannot mix their gametes. reproductive barriers, even completion.
though they are not isolated between closely related species.
by obvious physical barriers, Such behavioral rituals enable
such as mountain ranges. mate recognition—a way to
identify potential mates of the
Example: These two fly species same species.
in the genus Rhagoletis occur in
the same geographic areas, but Example: In North America, the Example: Blue-footed boobies, Example: The shells of two species
the apple maggot fly (Rhagoletis geographic ranges of the western inhabitants of the Galápagos, mate of snails in the genus Bradybaena
pomonella) feeds and mates on spotted skunk (Spilogale gracilis) only after a courtship display spiral in different directions:
hawthorns and apples (a) while (c) and the eastern spotted skunk unique to their species. Part of the Moving inward to the center, one
its close relative, the blueberry (Spilogale putorius) (d) overlap, “script” calls for the male to spirals in a counterclockwise
maggot fly (R. mendax), mates but S. gracilis mates in late high-step (e), a behavior that calls direction (f, left), the other in a
and lays its eggs only on summer and S. putorius mates in the female’s attention to his bright clockwise direction (f, right). As a
blueberries (b). late winter. blue feet. result, the snails’ genital openings
(indicated by arrows) are not
(a) (c) aligned, and mating cannot be
completed.

(e) (f)

(d) Video: Blue-Footed
(b) Boobies Courtship Ritual

506 Unit four  Mechanisms of Evolution

Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult

Gametic Isolation Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown

FERTILIZATION VIABLE,
FERTILE
OFFSPRING

Sperm of one species may not The genes of different parent Even if hybrids are vigorous, they Some first-generation hybrids
be able to fertilize the eggs of species may interact in ways that may be sterile. If the chromo- are viable and fertile, but when
another species. For instance, impair the hybrid’s development somes of the two parent species they mate with one another or
sperm may not be able to or survival in its environment. differ in number or structure, with either parent species,
survive in the reproductive meiosis in the hybrids may fail to offspring of the next generation
tract of females of the other produce normal gametes. Since are feeble or sterile.
species, or biochemical the infertile hybrids cannot
mechanisms may prevent the produce offspring when they
sperm from penetrating the mate with either parent species,
membrane surrounding the genes cannot flow freely between
other species’ eggs. the species.

Example: Gametic isolation Example: Some salamander Example: The hybrid offspring of a Example: Strains of cultivated rice
separates certain closely related subspecies of the genus Ensatina male donkey (i) and a female horse have accumulated different mutant
species of aquatic animals, such as live in the same regions and habitats, (j) is a mule (k), which is robust recessive alleles at two loci in the
sea urchins (g). Sea urchins release where they may occasionally but sterile. A “hinny” (not shown), course of their divergence from a
their sperm and eggs into the hybridize. But most of the hybrids do the offspring of a female donkey common ancestor. Hybrids
surrounding water, where they not complete development, and and a male horse, is also sterile. between them are vigorous and
fuse and form zygotes. It is those that do are frail (h). fertile (l, left and right), but plants
difficult for gametes of different (i) in the next generation that carry
species, such as the red and purple too many of these recessive alleles
urchins shown here, to fuse are small and sterile (l, center).
because proteins on the surfaces Although these rice strains are not
of the eggs and sperm bind very yet considered different species,
poorly to each other. they have begun to be separated
by postzygotic barriers.

(g) (h) (j) (l)

(k)

chapter 24  The Origin of Species 507

these barriers can be classified according to whether they ◀ Grizzly bear (U. arctos)
contribute to reproductive isolation before or after fertiliza- ▼ Polar bear (U. maritimus)
tion. Prezygotic barriers (“before the zygote”) block
fertilization from occurring. Such barriers typically act in ◀ Hybrid
one of three ways: by impeding members of different spe- “grolar bear”
cies from attempting to mate, by preventing an attempted
mating from being completed successfully, or by hinder- Figure 24.4  Hybridization between two species of bears
ing fertilization if mating is completed successfully. If a in the genus Ursus.
sperm cell from one species overcomes prezygotic barriers
and fertilizes an ovum from another species, a variety of subjective criteria; researchers may disagree on which struc-
postzygotic barriers (“after the zygote”) may contribute tural features distinguish a species.
to reproductive isolation after the hybrid zygote is formed.
Developmental errors may reduce survival among hybrid The ecological species concept defines a species in
embryos. Or problems after birth may cause hybrids to be terms of its ecological niche, the sum of how members of
infertile or decrease their chance of surviving long enough the species interact with the nonliving and living parts
to reproduce. of their environment (see Concept 54.1). For example,
two species of oak trees might differ in their size or in
Video: Prezygotic Barriers to Mating in Galápagos their ability to tolerate dry conditions, yet still occasion-
Finches by Peter and Rosemary Grant ally interbreed. Because they occupy different ecological
niches, these oaks would be considered separate species
Limitations of the Biological Species Concept even though they are connected by some gene flow. Unlike
the biological species concept, the ecological species con-
One strength of the biological species concept is that it cept can accommodate asexual as well as sexual species.
directs our attention to a way by which speciation can occur: It also emphasizes the role of disruptive natural selection
by the evolution of reproductive isolation. However, the as organisms adapt to different environments.
number of species to which this concept can be usefully
applied is limited. There is, for example, no way to evaluate In addition to those discussed here, more than 20 other
the reproductive isolation of fossils. The biological species species definitions have been proposed. The usefulness of
concept also does not apply to organisms that reproduce each definition depends on the situation and the research
asexually all or most of the time, such as prokaryotes. (Many questions being asked. For our purposes of studying how
prokaryotes do transfer genes among themselves, as we will species originate, the biological species concept, with its
discuss in Concept 27.2, but this is not part of their reproduc- focus on reproductive barriers, is particularly helpful.
tive process.) Furthermore, in the biological species concept,
species are designated by the absence of gene flow. But there Concept Check 24.1
are many pairs of species that are morphologically and eco-
logically distinct, and yet gene flow occurs between them. An 1. (a) Which species concept(s) could you apply to both
example is the grizzly bear (Ursus arctos) and polar bear (Ursus asexual and sexual species? (b) Which would be most
maritimus), whose hybrid offspring have been dubbed “grolar useful for identifying species in the field? Explain.
bears” (Figure 24.4). As we’ll discuss, natural selection can
cause such species to remain distinct even though some gene 2. WHAT IF? Suppose two bird species live in a forest
flow occurs between them. Because of the limitations to the and are not known to interbreed. One species feeds and
biological species concept, alternative species concepts are mates in the treetops and the other on the ground. But
useful in certain situations. in captivity, the birds can interbreed and produce viable,
fertile offspring. What type of reproductive barrier most
Other Definitions of Species likely keeps these species separate in nature? Explain.
For suggested answers, see Appendix A.
While the biological species concept emphasizes the
separateness of different species due to reproductive barriers,
several other definitions emphasize the unity within a species.
For example, the morphological species concept distin-
guishes a species by body shape and other structural features.
The morphological species concept can be applied to asexual
and sexual organisms, and it can be useful even without
information on the extent of gene flow. In practice, scien-
tists often distinguish species using morphological criteria.
A disadvantage of this approach, however, is that it relies on

508 unit four  Mechanisms of Evolution

Concept  24.2 small rodents may find a wide river or deep canyon a formi-
dable barrier.
Speciation can take place with
or without geographic separation Once geographic isolation has occurred, the separated
gene pools may diverge. Different mutations arise, and natu-
Having discussed what constitutes a unique species, let’s ral selection and genetic drift may alter allele frequencies in
return to the process by which such species arise from exist- different ways in the separated populations. Reproductive
ing species. We’ll describe this process by focusing on the isolation may then evolve as a by-product of the genetic
geographic setting in which gene flow is interrupted between divergence that results from selection or drift.
populations of the existing species—in allopatric speciation
the populations are geographically isolated, while in sympat- Figure 24.6 describes an example. On Andros Island, in the
ric speciation they are not (Figure 24.5). Bahamas, populations of the mosquitofish Gambusia hubbsi
colonized a series of ponds that later became isolated from one
Allopatric (“Other Country”) Speciation another. Genetic analyses indicate that little or no gene flow
currently occurs between the ponds. The environments of
In allopatric speciation (from the Greek allos, other, and these ponds are very similar except that some contain preda-
patra, homeland), gene flow is interrupted when a popula- tory fishes, while others do not. In ponds with predatory fishes,
tion is divided into geographically isolated subpopulations. selection has favored the evolution of a mosquitofish body
For example, the water level in a lake may subside, resulting shape that enables rapid bursts of speed (Figure 24.6). In ponds
in two or more smaller lakes that are now home to separated without predatory fishes, selection has favored a different body
populations (see Figure 24.5a). Or a river may change course shape, one that improves the ability to swim for long periods
and divide a population of animals that cannot cross it. of time. How have these different selective pressures affected
Allopatric speciation can also occur without geologic remodel- the evolution of reproductive barriers? Researchers studied
ing, such as when individuals colonize a remote area and their this question by bringing together mosquitofish from the two
descendants become geographically isolated from the parent types of ponds. They found that female mosquitofish prefer
population. The flightless cormorant shown in Figure 24.1 to mate with males whose body shape is similar to their own.
probably originated in this way from an ancestral flying This preference establishes a behavioral barrier to reproduction
species that reached the Galápagos Islands. between mosquitofish from ponds with predators and those

The Process of Allopatric Speciation Figure 24.6  Evolution in mosquitofish populations.
Different body shapes have evolved in mosquitofish populations from
How formidable must a geographic barrier be to promote allo- ponds with and without predators. These differences affect how
patric speciation? The answer depends on the ability of the quickly the fish can accelerate to escape and their survival rate when
organisms to move about. Birds, mountain lions, and coyotes exposed to predators.
can cross rivers and canyons—as can the windblown pollen of
pine trees and the seeds of some flowering plants. In contrast, In ponds with predatory fishes, In ponds without predatory
the mosquitofish‘s head is fishes, mosquitofish have a
Figure 24.5  The geography of speciation. streamlined and the tail is different body shape that
powerful, enabling rapid bursts favors long, steady swimming.
of speed.
(a) Differences in body shape

110Escape acceleration (m per s2) 1.0
Survival rate with predators
100 0.8

90 0.6
80
0.4
70

60 0.2

50 0.0
Source pond: Source pond: Source pond: Source pond:
predators no predators no
(a) Allopatric speciation. A pop- (b) Sympatric speciation. A present predators present predators
ulation forms a new species subset of a population forms
while geographically isolated a new species without (b) Differences in escape acceleration and survival
from its parent population. geographic separation.

chapter 24  The Origin of Species 509

from ponds without predators. Thus, as a by-product of selec- Figure 24.7
tion for avoiding predators, reproductive barriers have formed
in these allopatric populations. Inquiry  Can divergence of allopatric populations
lead to reproductive isolation?
Evidence of Allopatric Speciation
Experiment  A researcher divided a laboratory population of the
Many studies provide evidence that speciation can occur in fruit fly Drosophila pseudoobscura, raising some flies on a starch
allopatric populations. For example, laboratory studies show medium and others on a maltose medium. After one year (about
that reproductive barriers can develop when populations are 40 generations), natural selection resulted in divergent evolution:
isolated experimentally and subjected to different environ- Populations raised on starch digested starch more efficiently, while
mental conditions (Figure 24.7). those raised on maltose digested maltose more efficiently. The
researcher then put flies from the same or different populations in
Field studies indicate that allopatric speciation also can mating cages and measured mating frequencies. All flies used in the
occur in nature. Consider the 30 species of snapping shrimp mating preference tests were reared for one generation on a stan-
in the genus Alpheus that live off the Isthmus of Panama, dard cornmeal medium.
the land bridge that connects South and North America
(Figure 24.8). Fifteen of these species live on the Atlantic Initial population
side of the isthmus, while the other 15 live on the Pacific of fruit flies
side. Before the isthmus formed, gene flow could occur (Drosophila
between the Atlantic and Pacific populations of snapping
shrimp. Did the species on different sides of the isthmus pseudoobscura)
originate by allopatric speciation? Morphological and
genetic data group these shrimp into 15 pairs of sister species, Some flies raised Some flies raised on
pairs whose member species are each other’s closest relative. on starch medium maltose medium

Figure 24.8  Allopatric speciation in snapping shrimp Mating experiments
(Alpheus). The shrimp pictured are just 2 of the 15 pairs of sister after 40 generations
species that arose as populations were divided by the formation of the
Isthmus of Panama. The color-coded type indicates the sister species. Results  Mating patterns among populations of flies raised on
different media are shown below. When flies from “starch populations”
were mixed with flies from “maltose populations,” the flies tended to
mate with like partners. But in the control group (shown on the right),
flies from different populations adapted to starch were about as likely
to mate with each other as with flies from their own population; similar
results were obtained for control groups adapted to maltose.

Female Female
Starch Maltose
Starch Starch
population 1 population 2

A. formosus A. nuttingi Starch 22 9 Starchpopulation 2 population 1 18 15

Male Male

Maltose 8 20 Starch 12 15

ATLANTIC OCEAN

Number of matings Number of matings
in experimental group in control group

Isthmus of Panama Conclusion  In the experimental group, the strong preference of
“starch flies” and “maltose flies” to mate with like-adapted flies
PACIFIC OCEAN indicates that a reproductive barrier was forming between these fly
populations. Although this barrier was not absolute (some mating
between starch flies and maltose flies did occur), after 40 generations
reproductive isolation appeared to be increasing. This barrier may
have been caused by differences in courtship behavior that arose
as an incidental by-product of differing selective pressures as these
allopatric populations adapted to different sources of food.

Data from  D. M. B. Dodd, Reproductive isolation as a consequence of adaptive diver-
gence in Drosophila pseudoobscura, Evolution 43:1308–1311 (1989).

WHAT IF? Why were all flies used in the mating preference tests reared on
a standard medium (rather than on starch or maltose)?

A. panamensis A. millsae

510 unit four  Mechanisms of Evolution

In each of these 15 pairs, one of the sister species lives on examined reproductive isolation in geographically separated
the Atlantic side of the isthmus, while the other lives on salamander populations.
the Pacific side. This fact strongly suggests that the two
species arose as a consequence of geographic separation. Note that while geographic isolation prevents interbreed-
Furthermore, genetic analyses indicate that the Alpheus spe- ing between members of allopatric populations, physical sep-
cies originated from 9 to 3 million years ago, with the sister aration is not a biological barrier to reproduction. Biological
species that live in the deepest water diverging first. These reproductive barriers such as those described in Figure 24.3
divergence times are consistent with geologic evidence that are intrinsic to the organisms themselves. Hence, it is biologi-
the isthmus formed gradually, starting 10 million years ago, cal barriers that can prevent interbreeding when members of
and closing completely about 3 million years ago. different populations come into contact with one another.

The importance of allopatric speciation is also suggested Sympatric (“Same Country”) Speciation
by the fact that regions that are isolated or highly subdivided
by barriers typically have more species than do otherwise sim- In sympatric speciation (from the Greek syn, together), spe-
ilar regions that lack such features. For example, many unique ciation occurs in populations that live in the same geographic
plants and animals are found on the geographically isolated area (see Figure 24.5b). How can reproductive barriers form
Hawaiian Islands (we’ll return to the origin of Hawaiian spe- between sympatric populations while their members remain
cies in Concept 25.4). Field studies also show that reproduc- in contact with each other? Although such contact (and the
tive isolation between two populations generally increases as ongoing gene flow that results) makes sympatric speciation
the geographic distance between them increases, a finding less common than allopatric speciation, sympatric speciation
consistent with allopatric speciation. In the Scientific Skills can occur if gene flow is reduced by such factors as polyploidy,
Exercise, you will analyze data from one such study that sexual selection, and habitat differentiation. (Note that these
factors can also promote allopatric speciation.)

Scientific Skills Exercise

Identifying Independent and Dependent matings of each type between populations
Variables, Making a Scatter Plot, and (AB + BA). The table provides distance and
Interpreting Data reproductive isolation data for 27 pairs of
dusky salamander populations.
Does Distance Between Salamander Populations Increase
Their Reproductive Isolation?  Allopatric speciation begins Interpret The Data
when populations become geographically isolated, preventing
mating between individuals in different populations and thus stop- 1. State the researchers’ hypothesis, and identify the independent
ping gene flow. It is logical that as distance between populations and dependent variables in this study. Explain why the researchers
increases, so will their degree of reproductive isolation. To test this used four mating combinations for each pair of populations.
hypothesis, researchers studied populations of the dusky salamander
(Desmognathus ochrophaeus) living on different mountain ranges 2. Calculate the value of the reproductive isolation index if (a) all of
in the southern Appalachians. the matings within a population were successful, but none of the
matings between populations were successful; (b) salamanders are
How the Experiment Was Done  The researchers tested the equally successful in mating with members of their own popula-
reproductive isolation of pairs of salamander populations by leaving tion and members of another population.
one male and one female together and later checking the females
for the presence of sperm. Four mating combinations were tested for 3. Make a scatter plot to help you visualize any patterns that might
each pair of populations (A and B)—two within the same population indicate a relationship between the variables. Plot the independent
(female A with male A and female B with male B) and two between variable on the x-axis and the dependent variable on the y-axis.
populations (female A with male B and female B with male A). (For additional information about graphs, see the Scientific Skills
Review in Appendix F and the Study Area of MasteringBiology.)
Data from the Experiment  The researchers used an index of
reproductive isolation that ranged from a value of 0 (no isolation) to 4. Interpret your graph by (a) explaining in words any pattern
a value of 2 (full isolation). The proportion of successful matings for indicating a possible relationship between the variables and
each mating combination was measured, with 100% success = 1 and (b) hypothesizing the possible cause of such a relationship.
no success = 0. The reproductive isolation value for two populations
is the sum of the proportion of successful matings of each type within Instructors: A version of this Scientific Skills Exercise
populations (AA + BB) minus the sum of the proportion of successful can be assigned in MasteringBiology.

Geographic Distance (km) 15 32 40 47 42 Data from  S. G. Tilley, A. Verrell, and S. J. Arnold, Correspondence between sexual isola-
Reproductive Isolation Value 0.32 0.54 0.50 0.50 0.82 tion and allozyme differentiation: a test in the salamander Desmognathus ochrophaeus,
    Distance (continued) 137 150 165 189 219 Proceedings of the National Academy of Sciences USA 87:2715–2719 (1990).
    Isolation (continued) 0.50 0.57 0.91 0.93 1.5
62 63 81 86 107 107 115 137 147

0.37 0.67 0.53 1.15 0.73 0.82 0.81 0.87 0.87

239 247 53 55 62 105 179 169

1.22 0.82 0.99 0.21 0.56 0.41 0.72 1.15

chapter 24  The Origin of Species 511

Polyploidy Figure 24.10  One mechanism for allopolyploid speciation
in plants. Most hybrids are sterile because their chromosomes are not
A species may originate from an accident during cell divi- homologous and cannot pair during meiosis. However, such a hybrid may
be able to reproduce asexually. This diagram traces one mechanism that
sion that results in extra sets of chromosomes, a condition can produce fertile hybrids (allopolyploids) that are members of a new
species. The new species has a diploid chromosome number equal to the
called polyploidy. Polyploid speciation occasionally occurs sum of the diploid chromosome numbers of the two parent species.

in animals; for example, the gray tree frog Hyla versicolor

(see Figure 23.16) is thought to have originated in this way.

However, polyploidy is far more common in plants. In fact, Diploid Diploid
cell from cell from
botanists estimate that more than 80% of the plant species species A species B
2n = 6 2n = 4
alive today are descended from ancestors that formed by

polyploid speciation. Figure 24.9  Sympatric
Two distinct forms speciation by autopolyploidy.

of polyploidy have been Cell
observed in plant (and a division
few animal) populations. error
An autopolyploid (from
Normal gamete Normal gamete
the Greek autos, self) is an from species A from species B
n=3 n=2
individual that has more Diploid cell Tetraploid cell
than two chromosome sets 2n = 6 4n
that are all derived from a Meiosis

single species. In plants,

for example, a failure of 2n Sterile hybrid
cell division could double zygote
n=5
a cell’s chromosome num-

ber from the original num-

ber (2n) to a tetraploid 2n Cell from Mitotic or meiotic error
number (4n) (Figure 24.9). Gametes produced new species in a hybrid plant cell
by tetraploids doubles the chromosome
A tetraploid can pro- 4n number.
duce fertile tetraploid off-

spring by self-pollinating or by mating with other tetraploids.

In addition, the tetraploids are reproductively isolated from

2n plants of the original population, because the triploid (3n) Diploid cell from
new species:
offspring of such unions have reduced fertility. Thus, in just viable, fertile hybrid
(allopolyploid)
one generation, autopolyploidy can generate reproductive Figure 2n = 10
Walkthrough
isolation without any geographic separation.

A second form of polyploidy can occur when two different

species interbreed and produce hybrid offspring. Most such in abandoned parking lots and other urban sites. In 1950,
a new Tragopogon species was discovered near the Idaho-
hybrids are sterile because the set of chromosomes from one Washington border, a region where all three European species
also were found. Genetic analyses revealed that this new spe-
species cannot pair during meiosis with the set of chromo- cies, Tragopogon miscellus, is a hybrid of two of the European
species (Figure 24.11). Although the T. miscellus population
somes from the other species. However, an infertile hybrid grows mainly by reproduction of its own members, additional
episodes of hybridization between the parent species continue
may be able to propagate itself asexually (as many plants to add new members to the T. miscellus population. Later, scien-
tists discovered another new Tragopogon species, T. mirus—this
can do). In subsequent generations, various mechanisms one a hybrid of T. dubius and T. porrifolius (see Figure 24.11).
The Tragopogon story is just one of several well-studied exam-
can change a sterile hybrid into a fertile polyploid called an ples in which scientists have observed speciation in progress.

allopolyploid (Figure 24.10). The allopolyploids are fertile Many important agricultural crops—such as oats, cotton,
potatoes, tobacco, and wheat—are polyploids. For example,
when mating with each other but cannot interbreed with either the wheat used for bread, Triticum aestivum, is an allohexaploid
(six sets of chromosomes, two sets from each of three different
parent species; thus, they represent a new biological species. species). The first of the polyploidy events that eventually led

Although it can be challenging to study speciation in the

field, scientists have documented at least five new plant spe-

cies that have originated by polyploid speciation since 1850.

One of these examples involves the origin of a new species of

goatsbeard plant (genus Tragopogon) in the Pacific Northwest.

Tragopogon first arrived in the region when humans introduced

three European species in the early 1900s: T. pratensis, T. dubius,

and T. porrifolius. These three species are now common weeds

512 unit four  Mechanisms of Evolution

Figure 24.11  Allopolyploid speciation in Tragopogon. Figure 24.12
The gray boxes indicate the three parent species. The diploid
chromosome number of each species is shown in parentheses. Inquiry Does sexual selection in cichlids result
in reproductive isolation?
T. dubius
(12) Experiment  Researchers placed males and females of Pundamilia
pundamilia and P. nyererei together in two aquarium tanks, one with
natural light and one with a monochromatic orange lamp. Under
normal light, the two species are noticeably different in male breeding
coloration; under monochromatic orange light, the two species are
very similar in color. The researchers then observed the mate choices
of the females in each tank.

Hybrid species: Hybrid species: Normal light Monochromatic
T. miscellus T. mirus orange light
(24) (24)

P. pundamilia

T. pratensis T. porrifolius
(12) (12)

to modern wheat probably occurred about 8,000 years ago in P. nyererei
the Middle East as a spontaneous hybrid of an early cultivated
wheat species and a wild grass. Today, plant geneticists gener- Results  Under normal light, females of each species strongly
ate new polyploids in the laboratory by using chemicals that preferred males of their own species. But under orange light, females
induce meiotic and mitotic errors. By harnessing the evolu- of each species responded indiscriminately to males of both species.
tionary process, researchers can produce new hybrid species The resulting hybrids were viable and fertile.
with desired qualities, such as a hybrid that combines the high
yield of wheat with the hardiness of rye. Conclusion  The researchers concluded that mate choice by females
based on male breeding coloration can act as a reproductive barrier
Animation: Speciation by Changes in Ploidy that keeps the gene pools of these two species separate. Since the
species can still interbreed when this prezygotic behavioral barrier is
Sexual Selection breached in the laboratory, the genetic divergence between the spe-
cies is likely to be small. This suggests that speciation in nature has
There is evidence that sympatric speciation can also be driven occurred relatively recently.
by sexual selection. Clues to how this can occur have been
found in cichlid fish from one of Earth’s hot spots of animal Data from  O. Seehausen and J. J. M. van Alphen, The effect of male coloration on
speciation, East Africa’s Lake Victoria. This lake was once female mate choice in closely related Lake Victoria cichlids (Haplochromis nyererei
home to as many as 600 species of cichlids. Genetic data complex), Behavioral Ecology and Sociobiology 42:1–8 (1998).
indicate that these species originated within the last 100,000
years from a small number of colonizing species that arrived WHAT IF? Suppose that female cichlids living in the murky waters of
from other lakes and rivers. How did so many species—more a polluted lake could not distinguish colors well. In such waters, how might
than double the number of freshwater fish species known in the gene pools of these species change over time?
all of Europe—originate within a single lake?
Habitat Differentiation
One hypothesis is that subgroups of the original cichlid
populations adapted to different food sources and the resulting Sympatric speciation can also occur when a subpopulation
genetic divergence contributed to speciation in Lake Victoria. exploits a habitat or resource not used by the parent popula-
But sexual selection, in which (typically) females select males tion. Consider the North American apple maggot fly (Rhagoletis
based on their appearance (see Concept 23.4), may also have pomonella), a pest of apples. The fly’s original habitat was the
been a factor. Researchers have studied two closely related sym- native hawthorn tree (see Figure 24.3a), but about 200 years
patric species of cichlids that differ mainly in the coloration of ago, some populations colonized apple trees that had been
breeding males: Breeding Pundamilia pundamilia males have a introduced by European settlers. Apple maggot flies usually
blue-tinged back, whereas breeding Pundamilia nyererei males mate on or near their host plant. This results in a prezygotic
have a red-tinged back (Figure 24.12). Their results suggest barrier (habitat isolation) between populations that feed on
that mate choice based on male breeding coloration can act as apples and populations that feed on hawthorns. Furthermore,
a reproductive barrier that keeps the gene pools of these two because apples mature more quickly than hawthorn fruit,
species separate. natural selection has favored apple-feeding flies with rapid
development. These apple-feeding populations now show
temporal isolation from the hawthorn-feeding R. pomonella,
providing a second prezygotic barrier to gene flow between

chapter 24  The Origin of Species 513

the two populations. Researchers also have identified alleles Concept  24.3
that benefit the flies that use one host plant but harm the
flies that use the other host plant. Natural selection operating Hybrid zones reveal factors that
on these alleles has provided a postzygotic barrier to reproduc- cause reproductive isolation
tion, further limiting gene flow. Altogether, although the two
populations are still classified as subspecies rather than sepa- What happens if species with incomplete reproductive barriers
rate species, sympatric speciation appears to be well under way. come into contact with one another? One possible outcome is
the formation of a hybrid zone, a region in which members
Allopatric and Sympatric Speciation: of different species meet and mate, producing at least some
A Review offspring of mixed ancestry. In this section, we’ll explore
hybrid zones and what they reveal about factors that cause
Now let’s recap the processes by which new species form. the evolution of reproductive isolation.
In allopatric speciation, a new species forms in geographic
isolation from its parent population. Geographic isolation Patterns Within Hybrid Zones
severely restricts gene flow. Intrinsic barriers to reproduction
with members of the parent population may then arise as a Some hybrid zones form as narrow bands, such as the
by-product of genetic changes that occur within the isolated one depicted in Figure 24.13 for the yellow-bellied toad
population. Many different processes can produce such genetic (Bombina variegata) and its close relative, the fire-bellied toad
changes, including natural selection under different environ- (B. bombina). This hybrid zone, represented by the red line
mental conditions, genetic drift, and sexual selection. Once on the map, extends for 4,000 km but is less than 10 km wide
formed, reproductive barriers that arise in allopatric popula- in most places. The hybrid zone occurs where the higher-
tions can prevent interbreeding with the parent population altitude habitat of the yellow-bellied toad meets the lowland
even if the populations come back into contact. habitat of the fire-bellied toad. Across a given “slice” of the
zone, the frequency of alleles specific to yellow-bellied toads
Sympatric speciation, in contrast, requires the emergence typically decreases from close to 100% at the edge where
of a reproductive barrier that isolates a subset of a popula- only yellow-bellied toads are found to around 50% in the
tion from the remainder of the population in the same area. central portion of the zone to close to 0% at the edge where
Though rarer than allopatric speciation, sympatric speciation only fire-bellied toads are found.
can occur when gene flow to and from the isolated subpopu-
lation is blocked. This can occur as a result of polyploidy, What causes such a pattern of allele frequencies across a
a condition in which an organism has extra sets of chromo- hybrid zone? We can infer that there is an obstacle to gene
somes. Sympatric speciation also can result from sexual selec- flow—otherwise, alleles from one parent species would also be
tion. Finally, sympatric speciation can occur when a subset common in the gene pool of the other parent species. Are geo-
of a population becomes reproductively isolated because of graphic barriers reducing gene flow? Not in this case, since the
natural selection that results from a switch to a habitat or toads can move throughout the hybrid zone. A more important
food source not used by the parent population. factor is that hybrid toads have increased rates of embryonic
mortality and a variety of morphological abnormalities, includ-
Having reviewed the geographic context in which species ing ribs that are fused to the spine and malformed tadpole
originate, we’ll next explore in more detail what can happen mouthparts. Because the hybrids have poor survival and repro-
when new or partially formed species come into contact. duction, they produce few viable offspring with members of the
parent species. As a result, hybrid individuals rarely serve as a
HHMI Video: Anole Lizards: An Example stepping-stone from which alleles are passed from one species
of Speciation to the other. Outside the hybrid zone, additional obstacles to
gene flow may be provided by natural selection in the different
Concept Check 24.2 environments in which the parent species live.

1. Summarize key differences between allopatric and sympat- Hybrid zones typically are located wherever the habitats of
ric speciation. Which type of speciation is more common, the interbreeding species meet. Those regions often resemble
and why? a group of isolated patches scattered across the landscape—
more like the complex pattern of spots on a Dalmatian than
2. Describe two mechanisms that can decrease gene flow the continuous band shown in Figure 24.13. But regardless of
in sympatric populations, thereby making sympatric whether they have complex or simple spatial patterns, hybrid
speciation more likely to occur. zones form when two species lacking complete barriers to
reproduction come into contact. What happens when the
3. WHAT IF? Is allopatric speciation more likely to occur habitats of the interbreeding species change over time?
on an island close to a mainland or on a more isolated
island of the same size? Explain your prediction.

4. MAKE CONNECTIONS Review the process of meiosis in
Figure 13.8. Describe how an error during meiosis could
lead to polyploidy.

For suggested answers, see Appendix A.

514 unit four  Mechanisms of Evolution

Figure 24.13  A narrow hybrid zone for Bombina toads in Europe. The graph shows species-
specific allele frequencies across the width of the zone near Krakow, Poland, averaged over six genetic loci.
A value of 1.0 indicates that all individuals were yellow-bellied toads, 0 indicates that all individuals were fire-
bellied toads, and intermediate frequencies indicate that some individuals were of mixed ancestry.

Hybrid zone (red line)
occurs where the habitats
of the two species meet.

Fire-belliedFrequency of B. variegata-specific alleles Fire-bellied toad, Bombina bombina: lives at
toad range lower altitudes
Hybrid zone
Hybrid
Yellow-bellied zone
toad range

1.0

Yellow-bellied 0.75 Yellow-bellied Fire-bellied
toad, Bombina 0.5 toad range toad range
variegata: lives
at higher altitudes 0.25

? D oes the graph indicate that gene flow is spreading fire-bellied toad alleles
into the range of the yellow-bellied toad? Explain.
0
40 30 20 10 0 10 20
Distance from hybrid zone center (km)

Hybrid Zones and Environmental Change parent species to cope with changing environmental con-
ditions. This can occur when an allele found only in one
A change in environmental conditions can alter where the parent species is transferred first to hybrid individuals, and
habitats of interbreeding species meet. When this happens, then to the other parent species when hybrids breed with
an existing hybrid zone can move to a new location, or a the second parent species. Recent genetic analyses have
novel hybrid zone may form. shown that hybridization has been a source for such novel
genetic variation in various insect, bird, and plant species. In
For example, black-capped chickadees (Poecile atricapillus) the Problem-Solving Exercise, you can examine one such
and Carolina chickadees (P. carolinensis) interbreed in a narrow example: a case in which hybridization may have led to the
hybrid zone that runs from New Jersey to Kansas. Recent stud- transfer of insecticide-resistance alleles between mosquitoes
ies have shown that the location of this hybrid zone has shifted that transmit malaria.
northward as the climate has warmed. In another example, a
series of warm winters prior to 2003 enabled the southern fly- Hybrid Zones over Time
ing squirrel (Glaucomys volans) to expand northward into the
range of the northern flying squirrel, G. sabrinus. Previously, Studying a hybrid zone is like observing a naturally occur-
the ranges of these two species had not overlapped. Genetic ring experiment on speciation. Will the hybrids become
analyses showed that these flying squirrels began to hybridize reproductively isolated from their parents and form a new
where their ranges came into contact, thereby forming a novel species, as occurred by polyploidy in the goatsbeard plant of
hybrid zone induced by climate change. the Pacific Northwest? If not, there are three other common
outcomes for the hybrid zone over time: reinforcement of
Finally, note that a hybrid zone can be a source of novel
genetic variation that improves the ability of one or both

chapter 24  The Origin of Species 515

Problem-Solving Exercise

Is hybridization In this exercise, you will investigate whether alleles encoding resistance to insecticides have
promoting been transferred between closely related species of Anopheles.
insecticide
resistance in Your Approach The principle guiding your investigation is that DNA analyses can detect
mosquitoes that the transfer of resistance alleles between closely related mosquito species.
transmit malaria? To find out whether such transfers have occurred, you will analyze DNA
results from two species of mosquitoes that transmit malaria ( Anopheles
Malaria is a leading cause of human gambiae and A. coluzzii ) and from A. gambiae 3 A. coluzzii hybrids.
illness and mortality worldwide, with
200 million people infected and 600,000 Your Data Resistance to DDT and other insecticides in Anopheles is affected by a
deaths each year. In the 1960s, the inci- sodium channel gene, kdr. The r allele of this gene confers resistance, while
dence of malaria was reduced owing to the wild type (1/1) genotype is not resistant. Researchers sequenced the
the use of insecticides that killed mos- kdr gene from mosquitoes collected in Mali during three time periods:
quitoes in the genus Anopheles, which pre-2006 (2002 and 2004), 2006, and post-2006 (2009–2012). A. gambiae
transmit the disease from person to per- and A. coluzzii were collected during all three time periods, but their
son. But today, mosquitoes are becoming h­ ybrids only occurred in 2006, the first year that insecticide-treated bed
resistant to insecticides—causing a resur- nets were used to reduce the spread of malaria. A likely explanation is
gence in malaria. that the introduction of the treated bed nets may have briefly favored
hybrid individuals, which are usually at a selective disadvantage.

Observed numbers of mosquitoes by kdr genotype

1 /1 1 / r r / r

A. gambiae   3   5  2
  Pre-2006   8   8  7
  2006   3   3 57
  Post-2006

Hybrids  10   7  0
  2006

A. coluzzii 226   0  0
  Pre-2006  70   7  0
  2006  79 127 94
  Post-2006

Insecticide-treated bed nets have helped Your Analysis 1. How did the frequencies of kdr genotypes change over time in
reduce cases of malaria in many countries, A. gambiae? Describe a hypothesis that accounts for these observations.
but resistance to insecticides is rising in
mosquito populations. 2. How did the frequencies of kdr genotypes change over time in
A. coluzzii ? Describe a hypothesis that accounts for these observations.
Instructors: A version of this
Problem-Solving Exercise can be 3. Do these results indicate that hybridization can lead to the transfer
assigned in MasteringBiology. of adaptive alleles? Explain.

4. Predict how the transfer of the r allele to A. coluzzii populations
could affect the number of malaria cases.

barriers, fusion of species, or stability (Figure 24.14). Let’s As an example, let’s consider two species of European fly-
examine what studies suggest about these possibilities. catcher, the pied flycatcher (Ficedula hypoleuca) and the col-
lared flycatcher (Ficedula albicollis). In allopatric populations
Reinforcement: Strengthening Reproductive of these birds, males of the two species closely resemble one
Barriers another, while in sympatric populations, the males look very
different. Female flycatchers do not select males of the other
Hybrids often are less fit than members of their parent species when given a choice between males from sympatric
species. In such cases, natural selection should strengthen populations, but they frequently do make mistakes when
prezygotic barriers to reproduction, reducing the formation selecting between males from allopatric populations. Thus,
of unfit hybrids. Because this process involves reinforcing barriers to reproduction are stronger in birds from sympatric
reproductive barriers, it is called reinforcement. If rein- populations than in birds from allopatric populations, as you
forcement is occurring, a logical prediction is that barriers would predict if reinforcement were occurring. Similar results
to reproduction between species should be stronger for have been observed in a number of organisms, including
sympatric populations than for allopatric populations. fishes, insects, plants, and other birds.

516 unit four  Mechanisms of Evolution

Figure 24.14  Formation of a hybrid zone and possible outcomes for hybrids

over time. The thick gray arrows represent

the passage of time. 3 This population 4 Gene flow is
re-established in
1 Three 2 A barrier begins to diverge a hybrid zone.
populations to gene flow from the other
two populations.

of a species is established. 5 Possible outcomes for hybrids:
are connected
by gene flow. Reinforcement
(strengthening
Hybrid of reproductive
zone barriers—hybrids
gradually cease
to be formed)
OR

Gene flow Hybrid Fusion
Population individual (weakening of
reproductive
Barrier to barriers—the
gene flow two species fuse)
OR
WHAT IF? Predict what might happen if gene flow were re-established Stability
at step 3 in this process. (continued
production of
hybrid individuals)

Fusion: Weakening Reproductive Barriers Figure 24.15  Fusion: the breakdown of reproductive
barriers. Increasingly cloudy water in Lake Victoria over the past
Barriers to reproduction may be weak when two species meet several decades may have weakened reproductive barriers between
in a hybrid zone. Indeed, so much gene flow may occur that P. nyererei and P. pundamilia. In areas of cloudy water, the two
reproductive barriers weaken further and the gene pools of the species have hybridized extensively, causing their gene pools to fuse.
two species become increasingly alike. In effect, the speciation
process reverses, eventually causing the two hybridizing spe- Pundamilia nyererei Pundamilia pundamilia
cies to fuse into a single species.
Pundamilia ”turbid water,”
For example, genetic and morphological evidence hybrid offspring from a location
indicate that the recent loss of the large tree finch from with turbid water
the Galápagos island of Floreana resulted from extensive
hybridization with another finch species on that island. hybrid zones have also been observed in cases where the
Such a situation also may be occurring among Lake Victoria hybrids are selected against—an unexpected result.
cichlids. Many pairs of ecologically similar cichlid species
are reproductively isolated because the females of one spe- For example, hybrids continue to form in the Bombina
cies prefer to mate with males of one color, while females hybrid zone even though they are strongly selected against.
of the other species prefer to mate with males of a different
color (see Figure 24.12). Results from field and laboratory
studies indicate that murky waters caused by pollution have
reduced the ability of females to use color to distinguish
males of their own species from males of closely related
species. In some polluted waters, many hybrids have been
produced, leading to fusion of the parent species’ gene
pools and a loss of species (Figure 24.15).

Stability: Continued Formation of Hybrid
Individuals

Many hybrid zones are stable in the sense that hybrids con-
tinue to be produced. In some cases, this occurs because the
hybrids survive or reproduce better than members of either
parent species, at least in certain habitats or years. But stable

chapter 24  The Origin of Species 517

One explanation relates to the narrowness of the Bombina Patterns in the Fossil Record
hybrid zone (see Figure 24.13). Evidence suggests that mem-
bers of both parent species migrate into the zone from the The fossil record includes many episodes in which new spe-
parent populations located outside the zone, thus leading to cies appear suddenly in a geologic stratum, persist essentially
the continued production of hybrids. If the hybrid zone were unchanged through several strata, and then disappear. For
wider, this would be less likely to occur, since the center of example, there are dozens of species of marine invertebrates
the zone would receive little gene flow from distant parent that make their debut in the fossil record with novel mor-
populations located outside the hybrid zone. phologies, but then change little for millions of years before
becoming extinct. The term punctuated equilibria is used
Sometimes the outcomes in hybrid zones match our pre- to describe these periods of apparent stasis punctuated by
dictions (European flycatchers and cichlid fishes), and some- sudden change (Figure 24.16a). Other species do not show
times they don’t (Bombina). But whether our predictions a punctuated pattern; instead, they appear to have changed
are upheld or not, events in hybrid zones can shed light on more gradually over long periods of time (Figure 24.16b).
how barriers to reproduction between closely related species
change over time. In the next section, we’ll examine how What might punctuated and gradual patterns tell us about
interactions between hybridizing species can also provide how long it takes new species to form? Suppose that a species
a glimpse into the speed and genetic control of speciation. survived for 5 million years, but most of the morphological
changes that caused it to be designated a new species occurred
Concept Check 24.3 during the first 50,000 years of its existence—just 1% of its
total lifetime. Time periods this short (in geologic terms) often
1. What are hybrid zones, and why can they be viewed as cannot be distinguished in fossil strata, in part because the rate
“natural laboratories” in which to study speciation? of sediment accumulation may be too slow to separate layers
this close in time. Thus, based on its fossils, the species would
2. WHAT IF? Consider two species that diverged while seem to have appeared suddenly and then lingered with little
geographically separated but resumed contact before or no change before becoming extinct. Even though such a
reproductive isolation was complete. Predict the out- species may have originated more slowly than its fossils s­ uggest
come over time if the two species mated indiscrimi- (in this case taking up to 50,000 years), a punctuated p­ attern
nately and (a) hybrid offspring survived and reproduced indicates that speciation occurred relatively rapidly. For
more poorly than offspring from intraspecific matings ­species whose fossils changed much more gradually, we also
or (b) hybrid offspring survived and reproduced as well ­cannot tell exactly when a new biological species formed, since
as offspring from intraspecific matings. information about reproductive isolation does not fossilize.
For suggested answers, see Appendix A.
Figure 24.16  Two models for the tempo of speciation.
Concept  24.4 (a) Punctuated model. New species change most as they branch from a

Speciation can occur rapidly or parent species and then change little for the rest of their existence.
slowly and can result from changes
in few or many genes Time

Darwin faced many questions when he began to ponder (b) Gradual model. Species diverge from one another
that “mystery of mysteries”—speciation. He found answers more slowly and steadily over time.
to some of those questions when he realized that evolution
by natural selection helps explain both the diversity of life
and the adaptations of organisms (see Concept 22.2). But
biologists since Darwin have continued to ask fundamental
questions about speciation. How long does it take for new
species to form? And how many genes change when one
species splits into two? Answers to these questions are
also emerging.

The Time Course of Speciation

We can gather information about how long it takes new spe-
cies to form from broad patterns in the fossil record and from
studies that use morphological data (including fossils) or
molecular data to assess the time interval between speciation
events in particular groups of organisms.

518 unit four  Mechanisms of Evolution

However, it is likely that speciation in such groups occurred Figure 24.18
relatively slowly, perhaps taking millions of years.
Inquiry  How does hybridization lead to speciation
Interview with Stephen Jay Gould: An “architect” of the concept in sunflowers?
of punctuated equilibria
Experiment  Loren Rieseberg and his colleagues crossed the two
Speciation Rates parent sunflower species, H. annuus and H. petiolaris, to produce
experimental hybrids in the laboratory (for each gamete, only two
The existence of fossils that display a punctuated pattern of the n = 17 chromosomes are shown).
suggests that once the process of speciation begins, it can be
completed relatively rapidly—a suggestion supported by a H. annuus H. petiolarus
growing number of studies. gamete gamete

For example, rapid speciation appears to have produced Cell from F1 experimental
the wild sunflower Helianthus anomalus. Genetic evidence hybrid (4 of the 2n = 34
indicates that this species originated by the hybridization of chromosomes are shown)
two other sunflower species, H. annuus and H. petiolaris. The
hybrid species H. anomalus is ecologically distinct and repro- Note that in the first (F1) generation, each chromosome of the
ductively isolated from both parent species (Figure 24.17). experimental hybrids consisted entirely of DNA from one or the
Unlike the outcome of allopolyploid speciation, in which other parent species. The researchers then tested whether the F1 and
there is a change in chromosome number after hybridization, subsequent generations of experimental hybrids were fertile. They also
in these sunflowers the two parent species and the hybrid used species-specific genetic markers to compare the chromosomes
all have the same number of chromosomes (2n = 34). How, in the experimental hybrids with the chromosomes in the naturally
then, did speciation occur? To study this question, research- occurring hybrid H. anomalus.
ers performed an experiment designed to mimic events in Results  Although only 5% of the F1 experimental hybrids were
nature (Figure 24.18). Their results indicated that natural fertile, after just four more generations the hybrid fertility rose to
selection could produce extensive genetic changes in hybrid more than 90%. The chromosomes of individuals from this fifth
populations over short periods of time. These changes appear hybrid generation differed from those in the F1 generation (see
to have caused the hybrids to diverge reproductively from above) but were similar to those in H. anomalus individuals from
their parents and form a new species, H. anomalus. natural populations:

The sunflower example, along with the apple maggot fly, H. anomalus
Lake Victoria cichlid, and fruit fly examples discussed earlier,
suggests that new species can arise rapidly once divergence Chromosome 1
begins. But what is the total length of time between speciation
events? This interval consists of the time that elapses before Experimental hybrid
populations of a newly formed species start to diverge from H. anomalus
one another plus the time it takes for speciation to be com-
plete once divergence begins. It turns out that the total time Chromosome 2
between speciation events varies considerably. In a survey of
data from 84 groups of plants and animals, speciation inter- Experimental hybrid
vals ranged from 4,000 years (in cichlids of Lake Nabugabo, Comparison region containing H. annuus–specific marker
Comparison region containing H. petiolarus–specific marker
Figure 24.17  A hybrid sunflower species and its dry sand Conclusion  Over time, the chromosomes in the population of exper-
dune habitat. The wild sunflower Helianthus anomalus shown here imental hybrids became similar to the chromosomes of H. anomalus
originated via the hybridization of two other sunflowers, H. annuus individuals from natural populations. This suggests that the observed
and H. petiolaris, which live in nearby but moister environments. rise in the fertility of the experimental hybrids may have occurred as
selection eliminated regions of DNA from the parent species that were
not compatible with one another. Overall, it appeared that the initial
steps of the speciation process occurred rapidly and could be mimicked
in a laboratory experiment.

Data from  L. H. Rieseberg et al., Role of gene interactions in hybrid speciation: evidence
from ancient and experimental hybrids, Science 272:741–745 (1996).
WHAT IF?  The increased fertility of the experimental hybrids could have
resulted from natural selection for thriving under laboratory conditions.
Evaluate this alternative explanation for the result.

chapter 24  The Origin of Species 519

Uganda) to 40 million years (in some beetles). Overall, the time Figure 24.19  A locus that influences pollinator choice.
between speciation events averaged 6.5 million years and was Pollinator preferences provide a strong barrier to reproduction between
rarely less than 500,000 years. Mimulus lewisii and M. cardinalis. After transferring the M. lewisii allele
for a flower-color locus into M. cardinalis and vice versa, researchers
These data suggest that on average, millions of years may observed a shift in some pollinators’ preferences.
pass before a newly formed plant or animal species will itself
give rise to another new species. As you’ll read in Concept 25.4, (a) Typical Mimulus lewisii (b) M. lewisii with an M.
this finding has implications for how long it takes life on Earth cardinalis flower-color
to recover from mass extinction events. Moreover, the extreme allele
variability in the time it takes new species to form indicates
that organisms do not have an internal “speciation clock” that (c) Typical Mimulus cardinalis (d) M. cardinalis with an M.
causes them to produce new species at regular intervals. Instead, lewisii flower-color allele
speciation begins only after gene flow between populations is
interrupted, perhaps by changing environmental conditions WHAT IF If M. cardinalis individuals that had the M. lewisii yup allele were
or by unpredictable events, such as a storm that transports a planted in an area that housed both monkey flower species, how might the
few individuals to a new area. Furthermore, once gene flow is
interrupted, the populations must diverge genetically to such production of hybrid offspring be affected?
an extent that they become reproductively isolated—all before
other events cause gene flow to resume, possibly reversing the plants with the M. cardinalis yup allele received 68-fold more
speciation process (see Figure 24.15). visits from hummingbirds than did wild-type M. lewisii.
Similarly, M. cardinalis plants with the M. lewisii yup allele
Studying the Genetics of Speciation received 74-fold more visits from bumblebees than did wild-
type M. cardinalis. Thus, a mutation at a single locus can
Studies of ongoing speciation (as in hybrid zones) can reveal influence pollinator preference and hence contribute to
traits that cause reproductive isolation. By identifying the reproductive isolation in monkey flowers.
genes that control those traits, scientists can explore a funda-
mental question of evolutionary biology: How many genes In other organisms, the speciation process is influenced
influence the formation of new species? by larger numbers of genes and gene interactions. For
example, hybrid sterility between two subspecies of the fruit
In some cases, the evolution of reproductive isolation results fly Drosophila pseudoobscura results from gene interactions
from the effects of a single gene. For example, in Japanese among at least four loci, and postzygotic isolation in the
snails of the genus Euhadra, a change in a single gene results in sunflower hybrid zone discussed earlier is influenced by at
a mechanical barrier to reproduction. This gene controls the least 26 chromosome segments (and an unknown number
direction in which the shells spiral. When their shells spiral in of genes). Overall, studies suggest that few or many genes
different directions, the snails’ genitalia are oriented in a man- can influence the evolution of reproductive isolation and
ner that prevents mating (Figure 24.3f shows a similar exam- hence the emergence of a new species.
ple). Recent genetic analyses have uncovered other single genes
that cause reproductive isolation in fruit flies or mice.

A major barrier to reproduction between two closely related
species of monkey flower, Mimulus cardinalis and M. lewisii,
also appears to be influenced by a relatively small number of
genes. These two species are isolated by several prezygotic and
postzygotic barriers. Of these, one prezygotic barrier, pollina-
tor choice, accounts for most of the isolation: In a hybrid zone
between M. cardinalis and M. lewisii, nearly 98% of pollinator
visits were restricted to one species or the other.

The two monkey flower species are visited by different pol-
linators: Hummingbirds prefer the red-flowered M. cardinalis,
and bumblebees prefer the pink-flowered M. lewisii. Pollinator
choice is affected by at least two loci in the monkey flowers,
one of which, the “yellow upper,” or yup, locus, influences
flower color (Figure 24.19). By crossing the two parent species
to produce F1 hybrids and then performing repeated back-
crosses of these F1 hybrids to each parent species, researchers
succeeded in transferring the M. cardinalis allele at this locus
into M. lewisii, and vice versa. In a field experiment, M. lewisii

520 unit four  Mechanisms of Evolution

From Speciation to Macroevolution large-scale evolutionary changes as we begin our study of
macroevolution.
As you’ve seen, speciation may begin with differences as
small as the color on a cichlid’s back. However, as speciation Concept Check 24.4
occurs again and again, such differences can accumulate
and become more pronounced, eventually leading to the 1. Speciation can occur rapidly between diverging populations,
formation of new groups of organisms that differ greatly yet the time between speciation events is often more than a
from their ancestors (as in the origin of whales from ter- million years. Explain this apparent contradiction.
restrial mammals; see Figure 22.20). Moreover, as one
group of organisms increases in size by producing many 2. Summarize evidence that the yup locus acts as a prezygotic
new species, another group of organisms may shrink, los- barrier to reproduction in two species of monkey flowers. Do
ing species to extinction. The cumulative effects of many these results demonstrate that the yup locus alone controls
such speciation and extinction events have helped shape barriers to reproduction between these species? Explain.
the sweeping evolutionary changes that are documented
in the fossil record. In the next chapter, we turn to such 3. MAKE CONNECTIONS Compare Figure 13.12 with
Figure 24.18. What cellular process could cause the
hybrid chromosomes in Figure 24.18 to contain DNA
from both parent species? Explain.
For suggested answers, see Appendix A.

24 Chapter Review Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz,
Practice Tests, and more in the Study Area.

Summary of Key Concepts In sympatric speciation, a new species originates while
remaining in the same geographic area as the parent species.
Concept  24.1 Plant species (and, more rarely, animal species) have evolved
sympatrically through polyploidy. Sympatric speciation can
The biological species concept also result from sexual selection and habitat shifts.

emphasizes reproductive isolation ? Can factors that cause sympatric speciation also cause allopatric
speciation? Explain.
(pp. 505–508) VOCAB
SELF-QUIZ
goo.gl/6u55ks Concept  24.3
A biological species is a group of populations whose
individuals may interbreed and produce viable, fertile Hybrid zones reveal factors that cause
offspring with each other but not with members of other species. reproductive isolation (pp. 514–518)
The biological species concept emphasizes reproductive
isolation through prezygotic and postzygotic barriers that
separate gene pools. Many groups of organisms form hybrid zones in which mem-
bers of different species meet and mate, producing at least some
? Explain the role of gene flow in the biological species concept. offspring of mixed ancestry.

Concept  24.2 Many hybrid zones are stable, in that hybrid offspring continue
to be produced over time. In others, reinforcement strength-
Speciation can take place with or without ens prezygotic barriers to reproduction, thus decreasing the
geographic separation (pp. 509–514) formation of unfit hybrids. In still other hybrid zones, barriers
to reproduction may weaken over time, resulting in the fusion of
In allopatric speciation, gene flow is reduced when two the species’ gene pools (reversing the speciation process).
populations of one species become geographically separated
from each other. One or both populations may undergo evo- ? What factors can support the long-term stability of a hybrid zone if the
lutionary change during the period of separation, resulting in parent species live in different environments?
the establishment of barriers to reproduction.
Concept  24.4
Original
population Speciation can occur rapidly or slowly
and can result from changes in few
or many genes (pp. 518–521)

Allopatric Sympatric New species can form rapidly once divergence begins—but it
speciation speciation can take millions of years for that to happen. The time interval
between speciation events varies considerably, from a few thou-
sand years to tens of millions of years.

Researchers have identified particular genes involved in some
cases of speciation. Speciation can be driven by few or many genes.

? Is speciation something that happened only in the distant past, or are
new species continuing to arise today? Explain.

chapter 24  The Origin of Species 521

Test Your Understanding Evidence also indicates that the first polyploidy event was a
spontaneous hybridization of the early cultivated wheat species
Level 1: Knowledge/Comprehension T. monococcum and a wild Triticum grass species. Based on this
information, draw a diagram of one possible chain of events
1. The largest unit within which gene flow can that could have produced the allohexaploid T. aestivum.
readily occur is a
(A) population. (C) genus. Ancestral species:
(B) species. (D) hybrid.
PRACTICE
TEST
2. Males of different species of the fruit fly Drosophila goo.gl/CUYGKD
that live in the same parts of the Hawaiian Islands
have different elaborate courtship rituals. These rituals involve AA BB DD
fighting other males and making stylized movements that Triticum Wild Wild
attract females. What type of reproductive isolation does this monococcum Triticum T. tauschii
represent? (14) (14) (14)
(A) habitat isolation (C) behavioral isolation Product:
(B) temporal isolation (D) gametic isolation

3. According to the punctuated equilibria model, AA BB DD
(A) given enough time, most existing species will branch T. aestivum
gradually into new species. (bread wheat)
(B) most new species accumulate their unique features rela- (42)
tively rapidly as they come into existence, then change
little for the rest of their duration as a species.
(C) most evolution occurs in sympatric populations.
(D) speciation is usually due to a single mutation.

Level 2: Application/Analysis 9. WRITE ABOUT A THEME: INFORMATION  In sexually
reproducing species, each individual inherits DNA from both
4. Bird guides once listed the myrtle warbler and Audubon’s parent organisms. In a short essay (100–150 words), apply this
warbler as distinct species. Recently, these birds have been idea to what occurs when organisms of two species that have
classified as eastern and western forms of a single species, homologous chromosomes mate and produce (F1) hybrid
the yellow-rumped warbler. Which of the following pieces offspring. What percentage of the DNA in the F1 hybrids’
of evidence, if true, would be cause for this reclassification? chromosomes comes from each parent species? As the hybrids
(A) The two forms interbreed often in nature, and their mate and produce F2 and later-generation hybrid offspring,
offspring survive and reproduce well. describe how recombination and natural selection may affect
(B) The two forms live in similar habitats and have similar food whether the DNA in hybrid chromosomes is derived from one
requirements. parent species or the other.
(C) The two forms have many genes in common.
(D) The two forms are very similar in appearance. 10. SYNTHESIZE YOUR KNOWLEDGE

5. Which of the following factors would not contribute to
allopatric speciation?
(A) The separated population is small, and genetic drift occurs.
(B) The isolated population is exposed to different selection
pressures than the ancestral population.
(C) Different mutations begin to distinguish the gene pools of
the separated populations.
(D) Gene flow between the two populations is extensive.

6. Plant species A has a diploid chromosome number of 12. Plant
species B has a diploid number of 16. A new species, C, arises as
an allopolyploid from A and B. The diploid number for species
C would probably be
(A) 14. (B) 16. (C) 28. (D) 56.

Level 3: Synthesis/Evaluation Suppose that females of one population of strawberry poison
dart frogs (Dendrobates pumilio) prefer to mate with males that
7. EVOLUTION CONNECTION  Explain the biological basis for are orange-red in color. In a different population, females prefer
assigning all human populations to a single species. Can you males with yellow skin. Explain how such differences could arise
think of a scenario by which a second human species could and how they could affect the evolution of reproductive isola-
originate in the future? tion in allopatric versus sympatric populations.

8. SCIENTIFIC INQUIRY •  DRAW IT  In this chapter, you read that For selected answers, see Appendix A.
bread wheat (Triticum aestivum) is an allohexaploid, containing
two sets of chromosomes from each of three different parent For additional practice questions, check out the Dynamic Study
species. Genetic analysis suggests that the three species Modules in MasteringBiology. You can use them to study on
pictured following this question each contributed chromosome your smartphone, tablet, or computer anytime, anywhere!
sets to T. aestivum. (The capital letters here represent sets of
chromosomes rather than individual genes, and the diploid
chromosome number for each species is shown in parentheses.)

522 unit four  Mechanisms of Evolution

The History of Life on Earth 25

Figure 25.1  Would you have expected to find whale bones buried here?

Key Concepts A Surprise in the Desert

25.1 Conditions on early Earth made With its dry, wind-sculpted sands and searing heat, the Sahara Desert seems an
unlikely place to discover the bones of whales. But starting in the 1870s, researchers
the origin of life possible uncovered fossils of ancient whales at several locations that once were covered by an
ancient sea (Figure 25.1). For example, a nearly complete skeleton of Dorudon atrox,
25.2 The fossil record documents the an extinct whale that lived 35 million years ago, was discovered in a region that
came to be called Wadi Hitan, the “Valley of Whales.” Collectively, the whale fossils
history of life found in the Sahara were spectacular not only for where they were found, but also
for documenting early steps in the transition from life on land to life in the sea.
25.3 Key events in life’s history
Fossils discovered in other parts of the world tell a similar story: Past organisms
include the origins of unicellular were very different from those presently living. The sweeping changes in life on
and multicellular organisms and Earth as revealed by fossils illustrate macroevolution, the broad pattern of evolu-
the colonization of land tion above the species level. Examples of macroevolutionary change include the
emergence of terrestrial vertebrates through a series of speciation events, the impact
25.4 The rise and fall of groups of of mass extinctions on biodiversity, and the origin of key adaptations such as flight.

organisms reflect differences in Taken together, such changes provide a grand view of the evolutionary history
speciation and extinction rates of life. We’ll examine that history in this chapter, beginning with a discussion of
hypotheses regarding the origin of life. This is the most speculative topic of the entire
25.5 Major changes in body form unit, for no fossil evidence of that seminal episode exists. We will then turn to evi-
dence from the fossil record about major events in the history of life and the factors
can result from changes in the that have shaped the rise and fall of different groups of organisms over time.
sequences and regulation
of developmental genes

25.6 Evolution is not goal oriented

Fossil of Dorudon atrox, an ancient whale

When you see this blue icon, log in to MasteringBiology Get Ready for This Chapter
and go to the Study Area for digital resources.

523

Concept  25.1 Figure 25.2  Amino acid synthesis in a simulated volcanic
eruption. In addition to his classic 1953 study, Miller also conducted
Conditions on early Earth made an experiment simulating a volcanic eruption. In a 2008 reanalysis
the origin of life possible of those results, researchers found that far more amino acids were
produced under simulated volcanic conditions than were produced
Direct evidence of life on early Earth comes from fossils of in the conditions of the original 1953 experiment.
microorganisms that lived 3.5 billion years ago. But how did
the first living cells appear? Observations and experiments in Number of amino acids
chemistry, geology, and physics have led scientists to propose Mass of amino acids (mg)
one scenario that we’ll examine here. They hypothesize that 20 200
chemical and physical processes could have produced simple
cells through a sequence of four main stages: 10 100

1. The abiotic (nonliving) synthesis of small organic mol- 0 0
ecules, such as amino acids and nitrogenous bases 1953 2008 1953 2008

2. The joining of these small molecules into macromolecules, MAKE CONNECTIONS Explain how more than 20 amino acids could
such as proteins and nucleic acids have been produced in the 2008 experiment. (See Concept 5.4.)

3. The packaging of these molecules into protocells, by creating laboratory conditions comparable to those that
droplets with membranes that maintained an internal scientists at the time thought existed on early Earth (see
chemistry different from that of their surroundings Figure 4.2). His apparatus yielded a variety of amino acids
found in organisms today, along with other organic com-
4. The origin of self-replicating molecules that eventually pounds. Many laboratories have since repeated Miller’s clas-
made inheritance possible sic experiment using different recipes for the atmosphere,
some of which also produced organic compounds.
Though speculative, this scenario leads to predictions that
can be tested in the laboratory. In this section, we’ll examine However, some evidence suggests that the early atmo-
some of the evidence for each stage. sphere was made up primarily of nitrogen and carbon dioxide
and was neither reducing nor oxidizing (electron removing).
Synthesis of Organic Compounds Recent Miller/Urey-type experiments using such “neutral”
on Early Earth atmospheres have also produced organic molecules. In addi-
tion, small pockets of the early atmosphere, such as those near
Our planet formed 4.6 billion years ago, condensing from a the openings of volcanoes, may have been reducing. Perhaps
vast cloud of dust and rocks that surrounded the young sun. the first organic compounds formed near volcanoes. In 2008,
For its first few hundred million years, Earth was bombarded researchers used modern equipment to reanalyze molecules
by huge chunks of rock and ice left over from the formation that Miller had saved from one of his experiments. The 2008
of the solar system. The collisions generated so much heat paper found that numerous amino acids had formed under
that all of the available water was vaporized, preventing the conditions that simulated a volcanic eruption (Figure 25.2).
formation of seas and lakes.
Another hypothesis is that organic compounds were first
This massive bombardment ended 4 billion years ago, set- produced in deep-sea hydrothermal vents, areas on the
ting the stage for the origin of life. The first atmosphere had seafloor where heated water and minerals gush from Earth’s
little oxygen and was likely thick with water vapor, along interior into the ocean. Some of these vents, known as “black
with compounds released by volcanic eruptions, such as smokers,” release water so hot (300–400°C) that organic
nitrogen and its oxides, carbon dioxide, methane, ammonia, compounds formed there may have been unstable. But other
and hydrogen. As Earth cooled, the water vapor condensed deep-sea vents, called alkaline vents, release water that has
into oceans, and much of the hydrogen escaped into space. a high pH (9–11) and is warm (40–90°C) rather than hot, an
environment that may have been more suitable for the origin
During the 1920s, Russian chemist A. I. Oparin and British of life (Figure 25.3).
scientist J. B. S. Haldane independently hypothesized that
Earth’s early atmosphere was a reducing (electron-adding) Studies related to the volcanic-atmosphere and alkaline-
environment, in which organic compounds could have vent hypotheses show that the abiotic synthesis of organic
formed from simpler molecules. The energy for this synthesis molecules is possible under various conditions. Another
could have come from lightning and UV radiation. Haldane source of organic molecules may have been meteorites. For
suggested that the early oceans were a solution of organic example, fragments of the Murchison meteorite, a 4.5-billion-
molecules, a “primitive soup” from which life arose. year-old rock that landed in Australia in 1969, contain more
than 80 amino acids, some in large amounts. These amino
In 1953, Stanley Miller and Harold Urey, working at the
University of Chicago, tested the Oparin-Haldane hypothesis

524 Unit four  Mechanisms of Evolution

1 mm and a metabolic source of building blocks may have appeared
together in early protocells. The necessary conditions may
Figure 25.3  Did life originate have been met in vesicles, fluid-filled compartments enclosed
in deep-sea alkaline vents? The first by a membrane-like structure. Recent experiments show that
organic compounds may have arisen in abiotically produced vesicles can exhibit certain properties of
warm alkaline vents similar to this one life, including simple reproduction and metabolism, as well
from the 40,000-year-old “Lost City” as the maintenance of an internal chemical environment
vent field in the mid-Atlantic Ocean. different from that of their surroundings (Figure 25.4).
These vents contain hydrocarbons and
are full of tiny pores (inset) lined with For example, vesicles can form spontaneously when lipids
iron and other catalytic minerals. Early or other organic molecules are added to water. When this
oceans were acidic, so a pH gradient occurs, molecules that have both a hydrophobic region and
would have formed between the interior a hydrophilic region can organize into a bilayer similar to
of the vents and the surrounding ocean the lipid bilayer of a plasma membrane. Adding substances
water. Energy for the synthesis of such as montmorillonite, a soft mineral clay produced by the
organic compounds could have been weathering of volcanic ash, greatly increases the rate of vesicle
harnessed from this pH gradient. self-assembly (see Figure 25.4a). This clay, which is thought
to have been common on early Earth, provides surfaces on
acids cannot be contaminants from Earth because they consist which organic molecules become concentrated, increasing
of an equal mix of d and l isomers (see Figure 4.7). Organisms the likelihood that the molecules will react with each other
make and use only l isomers, with a few rare exceptions. and form vesicles. Abiotically produced vesicles can “repro-
Recent studies have shown that the Murchison meteorite duce” on their own (see Figure 25.4b), and they can increase
also contained other key organic molecules, including lipids,
simple sugars, and nitrogenous bases such as uracil. Figure 25.4  Features of abiotically produced vesicles.

Abiotic Synthesis of Macromolecules 0.4
Relative turbidity,
The presence of small organic molecules, such as amino acids an index of vesicle number Precursor molecules plus
and nitrogenous bases, is not sufficient for the emergence of life montmorillonite clay
as we know it. Every cell has many types of macromolecules, 0.2
including enzymes and other proteins and the nucleic acids Precursor
needed for self-replication. Could such macromolecules have molecules only
formed on early Earth? A 2009 study demonstrated that one key
step, the abiotic synthesis of RNA monomers, can occur spon- 0 60
taneously from simple precursor molecules. In addition, by 0 20 40
dripping solutions of amino acids or RNA nucleotides onto hot Time (minutes)
sand, clay, or rock, researchers have produced polymers of these
molecules. The polymers formed spontaneously, without the (a) Self-assembly. The presence of montmorillonite clay greatly
help of enzymes or ribosomes. Unlike proteins, the amino acid increases the rate of vesicle self-assembly.
polymers are a complex mix of linked and cross-linked amino
acids. Still, it is possible that such polymers acted as weak cata- Vesicle 1 μm
lysts for a variety of chemical reactions on early Earth. boundary

Protocells 20 μm

All organisms must be able to carry out both reproduction and (b) Reproduction. Vesicles can (c) Absorption of RNA. This
energy processing (metabolism). DNA molecules carry genetic divide on their own, as in this vesicle has incorporated
information, including the instructions needed to replicate vesicle ”giving birth” to montmorillonite clay particles
themselves accurately during reproduction. But DNA replica- smaller vesicles (LM). coated with RNA (orange).
tion requires elaborate enzymatic machinery, along with an
abundant supply of nucleotide building blocks provided by the MAKE CONNECTIONS Explain how molecules with both a hydrophobic
cell’s metabolism. This suggests that self-replicating molecules region and a hydrophilic region can self-assemble into a bilayer when in water.

(See Concept 5.3.)

chapter 25  The History of Life on Earth 525

in size (“grow”) without dilution of their contents. Vesicles duplication and other processes and as more properties of the
also can absorb montmorillonite particles, including those protocells became coded in genetic information. Once DNA
on which RNA and other organic molecules have become appeared, the stage was set for a blossoming of new forms
attached (see Figure 25.4c). Finally, experiments have shown of life—a change we see documented in the fossil record.
that some vesicles have a selectively permeable bilayer and
can perform metabolic reactions using an external source Interview with Jack Szostak: Studying the origin of life
of reagents—another important prerequisite for life. (see the interview before Chapter 22)

Self-Replicating RNA Concept Check 25.1

The first genetic material was most likely RNA, not DNA. RNA 1. What hypothesis did Miller test in his classic experiment?
plays a central role in protein synthesis, but it can also func- 2. How would the appearance of protocells have repre-
tion as an enzyme-like catalyst (see Concept 17.3). Such RNA
catalysts are called ribozymes. Some ribozymes can make sented a key step in the origin of life?
complementary copies of short pieces of RNA, provided that 3. MAKE CONNECTIONS In changing from an “RNA world”
they are supplied with nucleotide building blocks.
to today’s “DNA world,” genetic information must have
Natural selection on the molecular level has produced ribo- flowed from RNA to DNA. After reviewing Figures 17.4
zymes capable of self-replication in the laboratory. How does and 19.9, suggest how this could have occurred. Does such
this occur? Unlike double-stranded DNA, which takes the form a flow occur today?
of a uniform helix, single-stranded RNA molecules assume
a variety of specific three-dimensional shapes mandated by For suggested answers, see Appendix A.
their nucleotide sequences. In a given environment, RNA
molecules with certain nucleotide sequences may have shapes Concept  25.2
that enable them to replicate faster and with fewer errors than
other sequences. The RNA molecule with the greatest ability The fossil record documents
to replicate itself will leave the most descendant molecules. the history of life
Occasionally, a copying error will result in a molecule with a
shape that is even more adept at self-replication. Similar selec- Starting with the earliest traces of life, the fossil record opens
tion events may have occurred on early Earth. Thus, life as we a window into the world of long ago and provides glimpses of
know it may have been preceded by an “RNA world,” in which the evolution of life over billions of years. In this section, we’ll
small RNA molecules were able to replicate and to store genetic examine fossils as a form of scientific evidence: how fossils
information about the vesicles that carried them. form, how scientists date and interpret them, and what they
can and cannot tell us about changes in the history of life.
In 2013, Dr. Jack Szostak and colleagues succeeded in build-
ing a vesicle in which copying of a template strand of RNA The Fossil Record
could occur—a key step towards constructing a vesicle with
self-replicating RNA. On early Earth, a vesicle with such self- Sedimentary rocks are the richest source of fossils. As a result,
replicating, catalytic RNA would differ from its many neigh- the fossil record is based primarily on the sequence in which
bors that lacked such molecules. If that vesicle could grow, fossils have accumulated in sedimentary rock layers, called
split, and pass its RNA molecules to its “daughters,” the daugh- strata (see Figure 22.3). Useful information is also provided
ters would be protocells. Although the first such protocells by other types of fossils, such as insects preserved in amber
likely carried only limited amounts of genetic information, (fossilized tree sap) and mammals frozen in ice.
specifying only a few properties, their inherited characteristics
could have been acted on by natural selection. The most suc- The fossil record shows that there have been great changes
cessful of the early protocells would have increased in number in the kinds of organisms on Earth at different points in time
because they could exploit their resources effectively and pass (Figure 25.5). Many past organisms were unlike organisms
their abilities on to subsequent generations. living today, and many organisms that once were common
are now extinct. As we’ll see later in this section, fossils
Once RNA sequences that carried genetic information also document how new groups of organisms arose from
appeared in protocells, many additional changes would previously existing ones.
have been possible. For example, RNA could have provided
the template on which DNA nucleotides were assembled. As substantial and significant as the fossil record is, keep in
Double-stranded DNA is a more chemically stable repository mind that it is an incomplete chronicle of evolution. Many of
for genetic information than is the more fragile RNA. DNA Earth’s organisms did not die in the right place and time to be
also can be replicated more accurately. Accurate replication preserved as fossils. Of those fossils that were formed, many
was advantageous as genomes grew larger through gene were destroyed by later geologic processes, and only a fraction
of the others have been discovered. As a result, the known
526 Unit four  Mechanisms of Evolution fossil record is biased in favor of species that existed for a long
time, were abundant and widespread in certain kinds of envi-
ronments, and had hard shells, skeletons, or other parts that

Figure 25.5  Documenting the history of life. These fossils Present ▼ Rhomaleosaurus victor, a plesiosaur. These large
illustrate representative organisms from different points in time. Although marine reptiles were important predators from
prokaryotes and unicellular eukaryotes are shown only at the base of 200 million to 66 million years ago.
the diagram, these organisms continue to thrive today. In fact, most
organisms on Earth are unicellular.

▼ Dimetrodon, the largest known carnivore of its day, was more 100 million years ago
closely related to mammals than to reptiles. The spectacular “sail”
on its back may have functioned in temperature regulation or as an
ornament that served to attract mates.

1m ▼ Tiktaalik, an extinct
aquatic organism that
200 175 is the closest known
relative of the
four-legged vertebrates
that went on to
colonize land

0.5 m

300 270

4.5 cm 400 375 ▶ Hallucigenia, a
▲ Coccosteus cuspidatus, a placoderm (fishlike vertebrate) member of a
morphologically
that had a bony shield covering its head and front end diverse group of
animals found in
the Burgess Shale 1 cm
fossil bed in the ◀ Dickinsonia
Canadian Rockies
costata, a
510 500 2.5 cm member of the
Ediacaran biota,
an extinct group
of soft-bodied
organisms

▲ Some prokaryotes bind thin films 3,500 1,500 600 560 ▶ Tappania, a
of sediments together, producing unicellular
layered rocks called stromatolites. eukaryote
Present-day stromatolites are found thought to be
in a few shallow marine bays, such either an alga
as Shark Bay, Australia, shown here. or a fungus
▲ A section through a
fossilized stromatolite

chapter 25  The History of Life on Earth 527

facilitated their fossilization. Even with its limitations, however, older than that contain too little carbon-14 to be detected with
the fossil record is a remarkably detailed account of biological current techniques. Radioactive isotopes with longer half-lives
change over the vast scale of geologic time. Furthermore, as are used to date older fossils.
shown by the recently unearthed fossils of whale ancestors with
hind limbs (see Figures 22.19, 22.20, and 25.1), gaps in the fossil Determining the age of these older fossils in sedimentary
record continue to be filled by new discoveries. rocks can be challenging. Organisms do not use radioisotopes
with long half-lives, such as uranium-238, to build their bones
How Rocks and Fossils Are Dated or shells. In addition, sedimentary rocks are often composed
of sediments of differing ages. Although we cannot date these
Fossils are valuable data for reconstructing the history of life, older fossils directly, an indirect method can be used to infer
but only if we can determine where they fit in that unfold- the age of fossils that are sandwiched between two layers of
ing story. While the order of fossils in rock strata tells us the volcanic rock. As lava cools into volcanic rock, radioisotopes
sequence in which the fossils were laid down—their relative from the surrounding environment become trapped in the
ages—it does not tell us their actual ages. Examining the rela- newly formed rock. Some of the trapped radioisotopes have
tive positions of fossils is like peeling off layers of wallpaper in long half-lives, allowing geologists to estimate the ages of
an old house. You can infer the sequence in which the layers ancient volcanic rocks. If two volcanic layers surrounding
were applied, but not the year each layer was added. fossils are found to be 525 million and 535 million years old,
for example, then the fossils are roughly 530 million years old.
How can we determine the age of a fossil? One of the most
common techniques is radiometric dating, which is based The Origin of New Groups of Organisms
on the decay of radioactive isotopes (see Concept 2.2). In this
process, a radioactive “parent” isotope decays to a “daughter” Some fossils provide a detailed look at the origin of new
isotope at a characteristic rate. The rate of decay is expressed groups of organisms. Such fossils are central to our under-
by the half-life, the time required for 50% of the parent iso- standing of evolution; they illustrate how new features arise
tope to decay (Figure 25.6). Each type of radioactive isotope and how long it takes for such changes to occur. We’ll exam-
has a characteristic half-life, which is not affected by tempera- ine one such case here: the origin of mammals.
ture, pressure, or other environmental variables. For example,
carbon-14 decays relatively quickly; its half-life is 5,730 years. Along with amphibians and reptiles, mammals belong to
Uranium-238 decays slowly; its half-life is 4.5 billion years. the group of animals called tetrapods (from the Greek tetra, four,
and pod, foot), named for having four limbs. Mammals have
Fossils contain isotopes of elements that accumulated in the a number of unique anatomical features that fossilize readily,
organisms when they were alive. For example, a living organism allowing scientists to trace their origin. For example, the lower
contains the most common carbon isotope, carbon-12, as well jaw is composed of one bone (the dentary) in mammals but sev-
as a radioactive isotope, carbon-14. When the organism dies, it eral bones in other tetrapods. In addition, the lower and upper
stops accumulating carbon, and the amount of carbon-12 in its jaws in mammals hinge between a different set of bones than in
tissues does not change over time. However, the carbon-14 that other tetrapods. Mammals also have a unique set of three bones
it contains at the time of death slowly decays into another ele- that transmit sound in the middle ear, the hammer, anvil, and
ment, nitrogen-14. Thus, by measuring the ratio of carbon-14 stirrup, whereas other tetrapods have only one such bone, the
to carbon-12 in a fossil, we can determine the fossil’s age. This stirrup (see Concept 34.6). Finally, the teeth of mammals are
method works for fossils up to about 75,000 years old; fossils differentiated into incisors (for tearing), canines (for piercing),
and the multi-pointed premolars and molars (for crushing
Figure 25.6  Radiometric dating. In this diagram, each unit and grinding). In contrast, the teeth of other tetrapods usually
of time represents one half-life of a radioactive isotope. consist of a row of undifferentiated, single-pointed teeth.

Fraction of parent Accumulating As detailed in Figure 25.7, the fossil record shows that
isotope remaining “daughter” the unique features of mammalian jaws and teeth evolved
1 2 isotope gradually over time, in a series of steps. As you study
Figure 25.7, bear in mind that it includes just a few examples
Remaining 14 of the fossil skulls that document the origin of mammals. If
“parent” all the known fossils in the sequence were arranged by shape
isotope 18 1 16 and placed side by side, their features would blend smoothly
from one group to the next. Some of these fossils would reflect
12 3 4 how the features of a group that dominates life today, the
Time (half-lives) mammals, gradually arose in a previously existing group, the
cynodonts. Others would reveal side branches on the tree of
DRAW IT Relabel the x-axis of this graph in years to illustrate the radioactive life—groups of organisms that thrived for millions of years
decay of uranium-238 (half-life = 4.5 billion years). but ultimately left no descendants that survive today.

528 Unit four  Mechanisms of Evolution

Figure 25.7  Exploring The Origin of Mammals

Over the course of 120 million years, mammals originated OTHER Reptiles
gradually from a group of tetrapods called synapsids. Shown TETRAPODS (including
here are a few of the many fossil organisms whose morphological dinosaurs and birds)
features represent intermediate steps between living mammals Cynodonts †Dimetrodon
and their early synapsid ancestors. The evolutionary context of the Therapsids
origin of mammals is shown in the tree diagram at right (the dagger †Very late (non-
symbol † indicates extinct lineages). Synapsids mammalian)
cynodonts
Key to skull bones Dentary
Squamosal Mammals
Articular
Quadrate

Synapsid (300 mya) Temporal
fenestra
Early synapsids had multiple bones in the lower jaw and single-pointed
teeth. The jaw hinge was formed by the articular and quadrate bones. Early Hinge
synapsids also had an opening called the temporal fenestra behind the eye
socket. Powerful cheek muscles for closing the jaws probably passed through Temporal
the temporal fenestra. Over time, this opening enlarged and moved in front of fenestra
the hinge between the lower and upper jaws, thereby increasing the power and
precision with which the jaws could be closed (much as moving a doorknob
away from the hinge makes a door easier to close).

Therapsid (280 mya)

Later, a group of synapsids called therapsids appeared. Therapsids had large
dentary bones, long faces, and the first examples of specialized teeth, large
canines. These trends continued in a group of therapsids called cynodonts.

Early cynodont (260 mya) Hinge Canine tooth
Temporal
In early cynodont therapsids, the dentary was the largest bone in the lower fenestra
jaw, the temporal fenestra was large and positioned forward of the jaw hinge, (partial view)
and teeth with several cusps first appeared (not visible in the diagram). As in
earlier synapsids, the jaw had an articular-quadrate hinge.

Later cynodont (220 mya) Hinge

Later cynodonts had teeth with complex cusp patterns, and their lower and Original hinge
upper jaws hinged in two locations: They retained the original articular- New hinge
quadrate hinge and formed a new, second hinge between the dentary and
squamosal bones. (The temporal fenestra is not visible in this or the below Hinge
cynodont skull at the angles shown.)

Very late cynodont (195 mya)

In some very late (nonmammalian) cynodonts and early mammals, the original
articular-quadrate hinge was lost, leaving the dentary-squamosal hinge as the only
hinge between the lower and upper jaws, as in living mammals. The articular and
quadrate bones migrated into the ear region (not shown), where they functioned
in transmitting sound. In the mammal lineage, these two bones later evolved
into the familiar hammer (malleus) and anvil (incus) bones of the ear.

chapter 25  The History of Life on Earth 529

Concept Check 25.2 first three eons—the Hadean, Archaean, and Proterozoic—
together lasted about 4 billion years. The Phanerozoic eon,
1. Describe an example from the fossil record that shows roughly the last half billion years, encompasses most of the
how life has changed over time. time that animals have existed on Earth. It is divided into
three eras: the Paleozoic, Mesozoic, and Cenozoic. Each era
2. WHAT IF? Your measurements indicate that a fossilized represents a distinct age in the history of Earth and its life.
skull you unearthed has a carbon-14/carbon-12 ratio about For example, the Mesozoic era is sometimes called the “age of
1⁄16 that of the skulls of present-day animals. What is the reptiles” because of its abundance of reptilian fossils, includ-
approximate age of the fossilized skull? ing those of dinosaurs. The boundaries between the eras cor-
respond to major extinction events, when many forms of life
For suggested answers, see Appendix A. disappeared and were replaced by forms that evolved from
the survivors.
Concept  25.3
As we’ve seen, the fossil record provides a sweeping over-
Key events in life’s history include the view of the history of life over geologic time. Here we will focus
origins of unicellular and multicellular on a few major events in that history, returning to study the
organisms and the colonization of land details in Unit Five. Figure 25.8 will help you visualize how
long ago these key events occurred against the vast backdrop
The study of fossils has helped geologists establish a geologic of geologic time.
record: a standard time scale that divides Earth’s history
into four eons and further subdivisions (Table 25.1). The

Figure 25.8  Visualizing the Scale of Geologic Time

Geologic time is so vast that it can be dif cult to visualize Instructors: Additional questions related
to this Visualizing Figure can be assigned
when key events in the history of life on Earth occurred. in MasteringBiology.

This gure introduces two common representations

that help place the timing of those events Present Origin of Geologic time is represented here
in context: a countdown timer Humans as a timer that, moving clockwise
and a horizontal time line. from the top, “counts down” from
Earth’s origin (4.6 billion years
Using the analogy of a timer solar system ago) to the present.
that begins with the origin of and Earth
Earth and counts down for one
hour, we can relate the relative Colonization Phanerozoic
timing and duration of events of land Cenozoic Hadean 1 Using the analogy of a
that occurred billions of years Animals Mesozoic one-hour countdown timer,

ago to a familiar time scale. On Paleozoic when did prokaryotes
a one-hour time scale, animals
originated about 9 minutes ago, 4.5 originate? When did the
while humans appeared less .5 4
Prokaryotes colonization of land occur?

than 0.2 seconds ago. 1 Billions of 3.5
years ago
Archaean
Multicellular Proterozoic 1.5 3 This diagram shows all of geologic
eukaryotes 2 2.5 time to scale on an unbroken time
This diagram “uncoils” the timer to line, but it's often necessary to
represent life’s history on a horizontal time "break" the time line to limit the
line. Time runs from left to right, from size of a figure. Hatch marks are
4.6 billion years ago to the present. The often used to
color-coding will help you relate these represent this
diagrams to each other and to Table 25.1. interruption; see
Origin of Single-celled Atmospheric Figure 25.11 mHaartkchs
solar system eukaryotes oxygen for an example.
and Earth
Prokaryotes Atmospheric oxygen

Hadean Archaean

4.5 4 3.5 3 2.5
Billions of years ago

530 Unit four  Mechanisms of Evolution

Table 25.1 The Geologic Record

Eons Era Period Epoch Age Some Important
(duration (Millions of Events in the
not to scale) Years Ago) History of Life

Cenozoic Quaternary Holocene 0.01 Historical time
Neogene Pleistocene 2.6 Ice ages; origin of genus Homo
Pliocene 5.3 Appearance of bipedal human ancestors
Paleogene Miocene Continued radiation of mammals and
23 angiosperms; earliest direct human ancestors
Cretaceous Oligocene
Jurassic 34 Origins of many primate groups
Triassic Eocene
56 Angiosperm dominance increases; continued
Paleocene radiation of most present-day mammalian orders
66 Major radiation of mammals, birds,
and pollinating insects
Mesozoic 145
201 Flowering plants (angiosperms) appear and diversify; many groups of
Phan- 252 organisms, including most dinosaurs, become extinct at end of period
erozoic
Gymnosperms continue as dominant
Permian 299 plants; dinosaurs abundant and diverse
Carboniferous 359 Cone-bearing plants (gymnosperms) dominate landscape;
dinosaurs evolve and radiate; origin of mammals
Paleozoic Devonian 419 Radiation of reptiles; origin of most
Silurian present-day groups of insects; extinction of
Proter- Neo- Ordovician 444 many marine and terrestrial organisms at end of period
ozoic proterozoic Cambrian Extensive forests of vascular plants form; first seed plants appear;
Ediacaran 485 origin of reptiles; amphibians dominant

Archaean 541 Diversification of bony fishes; first tetrapods
Hadean and insects appear
635
1,000 Diversification of early vascular plants
1,800
2,500 Marine algae abundant; colonization of land by diverse
2,700 fungi, plants, and animals
3,500 Sudden increase in diversity of many animal phyla
4,000 (Cambrian explosion)
Approx. 4,600 Diverse algae and soft-bodied invertebrate
animals appear

Oldest fossils of eukaryotic cells appear

Concentration of atmospheric oxygen begins to increase
Oldest fossils of cells (prokaryotes) appear
Oldest known rocks on Earth’s surface
Origin of Earth

2 Once life began, what types of organisms lived on Earth for Present
the next two billion years? Where did these organisms live? Humans

Multicellular eukaryotes Animals Colonization of land

Single-celled eukaryotes

Proterozoic Paleozoic Mesozoic Ceno-
zoic

2 1.5 1 .5

Billions of years ago Phanerozoic

chapter 25  The History of Life on Earth 531

The First Single-Celled Organisms but then shot up relatively rapidly to between 1% and 10%
of its present level. This “oxygen revolution” had an enor-
Present The earliest direct evidence of life, mous impact on life. In some of its chemical forms, oxygen
attacks chemical bonds and can inhibit enzymes and damage
dating from 3.5 billion years ago, cells. As a result, the rising concentration of atmospheric O2
probably doomed many prokaryotic groups. Some species
4 comes from fossilized stromatolites survived in habitats that remained anaerobic, where we find
1 Billions of (see Figure 25.5). Stromatolites their descendants living today (see Concept 27.4). Among
are layered rocks that form when other survivors, diverse adaptations to the changing atmo-
years ago certain prokaryotes bind thin films of sphere evolved, including cellular respiration, which uses
23 O2 in the process of harvesting the energy stored in organic
molecules.
Prokaryotes
The rise in atmospheric O2 levels left a huge imprint on the
sediment together. Stromatolites and other early prokaryotes history of life. A few hundred million years later, another fun-
damental change occurred: the origin of the eukaryotic cell.
were Earth’s sole inhabitants for about 1.5 billion years. As we

will see, these prokaryotes transformed life on our planet.

Photosynthesis and the Oxygen Revolution

Present Most atmospheric oxygen gas (O2) is of
biological origin, produced during the

4 water-splitting step of photosynthesis. The First Eukaryotes
1 Billions of When oxygenic photosynthesis
first evolved—in photosynthetic
years ago
23

Atmospheric
oxygen Present
prokaryotes— the free O2 it produced The oldest widely accepted fossils of

probably dissolved in the surrounding water until it reached a eukaryotic organisms are 1.8 billion

high enough concentration to react with elements dissolved 4 years old. Recall that eukaryotic cells
1 Billions of have more complex organization
in water, including iron. This would have caused the iron to than prokaryotic cells: Eukaryotic
years ago cells have a nuclear envelope, mito-
precipitate as iron oxide, which accumulated as sediments. 23

These sediments were compressed into banded iron format­ ions, Single-celled
eukaryotes

red layers of rock containing iron oxide that are a source of chondria, endoplasmic reticulum, and other internal structures

iron ore today. Once all of the dissolved iron had precipitated, that prokaryotes lack. Also, unlike prokaryotic cells, eukaryotic

additional O2 dissolved in the water until the seas and lakes cells have a well-developed cytoskeleton, a feature that enables
became saturated with O2. After this occurred, the O2 finally
began to “gas out” of the water and enter the atmosphere. This eukaryotic cells to change their shape and thereby surround

and engulf other cells.

change left its mark in the rusting of iron-rich terrestrial rocks, a How did the eukaryotes evolve from their prokaryotic

process that began about 2.7 billion years ago. This chronology ancestors? Current evidence indicates that the eukaryotes

implies that bacteria similar to today’s cyanobacteria (oxygen- originated by endosymbiosis when a prokaryotic cell

releasing, photosynthetic bacteria) originated before 2.7 billion engulfed a small cell that would evolve into an organelle

years ago. found in all eukaryotes, the mitochondrion. The small,

As shown in Figure 25.9, the amount of atmospheric O2 engulfed cell is an example of an endosymbiont, a cell that
increased gradually from about 2.7 to 2.4 billion years ago,
lives within another cell, called the host cell. The prokaryotic

Figure 25.9  The rise of atmospheric oxygen. Chemical ancestor of the mitochondrion probably entered the host
analyses of ancient rocks have enabled this reconstruction of
atmospheric oxygen levels during Earth’s history. cell as undigested prey or an internal parasite. Though such a

process may seem unlikely, scientists have directly observed

cases in which endosymbionts that began as prey or parasites

Atmospheric O2 1,000 developed a mutually beneficial relationship with the host in
(percent of present-day levels; log scale)
100 as little as five years.

By whatever means the relationship began, we can hypoth-

10 esize how the symbiosis could have become beneficial. For

1 example, in a world that was becoming increasingly aerobic,

a host that was itself an anaerobe would have benefited from

0.1 endosymbionts that could make use of the oxygen. Over time,
0.01
0.001 “Oxygen the host and endosymbionts would have become a single
revolution”
organism, its parts inseparable. Although all eukaryotes have

mitochondria or remnants of these organelles, they do not all

have plastids (a general term for chloroplasts and related organ-

0.0001 elles). Thus, the serial endosymbiosis hypothesis supposes

4 321 0 that mitochondria evolved before plastids through a sequence
Time (billions of years ago)
of endosymbiotic events. As shown in Figure 25.10, both

532 Unit four  Mechanisms of Evolution

Figure 25.10  A hypothesis for the origin of mitochondria A great deal of evidence supports the endosymbiotic origin
and plastids through serial endosymbiosis. The proposed host of mitochondria and plastids:
was an archaean or a close relative of the archaeans. The proposed
ancestors of mitochondria were aerobic, heterotrophic bacteria, while The inner membranes of both organelles have enzymes
the proposed ancestors of plastids were photosynthetic bacteria. In this and transport systems that are homologous to those
figure, the arrows represent change over evolutionary time. found in the plasma membranes of living bacteria.

Cytoplasm Infolding Mitochondria and plastids replicate by a splitting process
DNA of plasma that is similar to that of certain bacteria. In addition,
membrane each of these organelles contains circular DNA molecules
that, like the chromosomes of bacteria, are not associated
Ancestral Plasma with histones or large amounts of other proteins.
prokaryote membrane
As might be expected of organelles descended from free-
Engulfed Endoplasmic Nucleus living organisms, mitochondria and plastids also have
aerobic reticulum the cellular machinery (including ribosomes) needed to
bacterium Nuclear transcribe and translate their DNA into proteins.
envelope
Finally, in terms of size, RNA sequences, and sensitivity to
certain antibiotics, the ribosomes of mitochondria and
plastids are more similar to bacterial ribosomes than they
are to the cytoplasmic ribosomes of eukaryotic cells.

In Chapter 28, we’ll return to the origin of eukaryotes, focus-
ing on what genomic data have revealed about the prokaryotic
lineages that gave rise to the host and endosymbiont cells.

Mitochondrion Cell with nucleus The Origin of Multicellularity
and endomembrane
system An orchestra can play a greater variety of musical compositions
than a violin soloist can; the increased complexity of the orches-
Ancestral tra makes more variations possible. Likewise, the origin of struc-
eukaryote turally complex eukaryotic cells sparked the evolution of greater
(a heterotroph) morphological diversity than was possible for the simpler pro-
karyotic cells. After the first eukaryotes appeared, a great range
of unicellular forms evolved, giving rise to the diversity of single-
celled eukaryotes that continue to flourish today. Another wave
of diversification also occurred: Some single-celled eukaryotes
gave rise to multicellular forms, whose descendants include a
variety of algae, plants, fungi, and animals.

Engulfed Early Multicellular Eukaryotes
photosynthetic
bacterium Present The oldest known fossils of
Plastid
multicellular eukaryotes that can
Multicellular 4 be resolved taxonomically are of
eukaryotes 1 Billions of relatively small red algae that lived
1.2 billion years ago; even older
years ago
23

fossils, dating to 1.8 billion years

Ancestral photosynthetic ago, may also be of small, multicellular eukaryotes. Larger
eukaryote
and more diverse multicellular eukaryotes do not appear in

Figure Walkthrough the fossil record until about 600 million years ago (see Figure

25.5). These fossils, referred to as the Ediacaran biota, were of

soft-bodied organisms—some over 1 m long—that lived from

mitochondria and plastids are thought to have descended from 635 to 541 million years ago. The Ediacaran biota included
bacterial cells. The original host—the cell that engulfed the bac-
terium whose descendants gave rise to the mitochondrion—is both algae and animals, along with various organisms of
thought to have been an archaean or a close relative of the
archaeans. unknown taxonomic affinity.

The rise of large eukaryotes in the Ediacaran period rep-

resents an enormous change in the history of life. Before

chapter 25  The History of Life on Earth 533

that time, Earth was a microbial world: Its only inhabitants that. In a relatively short period of time (10 million years),
were single-celled prokaryotes and eukaryotes, along with predators over 1 m in length emerged that had claws and
an assortment of microscopic, multicellular eukaryotes. other features for capturing prey; simultaneously, new defen-
As the diversification of the Ediacaran biota came to a close sive adaptations, such as sharp spines and heavy body armor,
about 541 million years ago, the stage was set for another, appeared in their prey (see Figure 25.5).
even more spectacular burst of evolutionary change:
the “Cambrian explosion.” Although the Cambrian explosion had an enormous
impact on life on Earth, it appears that many animal phyla
The Cambrian Explosion originated long before that time. Recent DNA analyses suggest
that sponges had evolved by 700 million years ago; such anal-
Present Many present-day animal phyla yses also indicate that the common ancestor of arthropods,
chordates, and other animal phyla that radiated during the
Animals appear suddenly in fossils formed Cambrian explosion lived 670 million years ago. Researchers
have unearthed 710-million-year-old sediments containing
4 535–525 million years ago, early in the steroids indicative of a particular group of sponges—a finding
1 Billions of Cambrian period. This phenomenon that supports the molecular data. In contrast, the oldest fossil
is referred to as the Cambrian assigned to an extant animal phylum is that of the mollusc
years ago Kimberella, which lived 560 million years ago. Overall, molec-
23 ular and fossil data indicate that the Cambrian explosion had
a “long fuse”—at least 25 million years long based on the age
explosion. Fossils of several animal of Kimberella fossils, and over 100 million years long based on
some DNA analyses.
groups—sponges, cnidarians (sea anemones and their

relatives), and molluscs (snails, clams, and their relatives)—

appear in even older rocks dating from the late Proterozoic

(Figure 25.11).

Prior to the Cambrian explosion, all large animals were

soft-bodied. The fossils of large pre-Cambrian animals reveal

little evidence of predation. Instead, these animals appear to

have been grazers (feeding on algae), filter feeders, or scav- The Colonization of Land

engers, not hunters. The Cambrian explosion changed all of Present The colonization of land was another
Colonization milestone in the history of life. There
of land is fossil evidence that cyanobacteria

Figure 25.11  Appearance of selected animal groups. The 4 and other photosynthetic prokaryotes
white bars indicate earliest appearances of these animal groups in the 1 Billions of coated damp terrestrial surfaces well
fossil record.
years ago
23

over a billion years ago. However,

Sponges larger forms of life, such as fungi,
Cnidarians
plants, and animals, did not begin to colonize land until
Echinoderms
Chordates about 500 million years ago. This gradual evolutionary

venture out of aquatic environments was associated with

adaptations that made it possible to reproduce on land and

that helped prevent dehydration. For example, many land

plants today have a vascular system for transporting materials

internally and a waterproof coating of wax on their leaves

Brachiopods that slows the loss of water to the air. Early signs of these

adaptations were present 420 million years ago, at which time

Annelids small plants (about 10 cm high) existed that had a vascular

system but lacked true roots or leaves. By 40 million years

Molluscs later, plants had diversified greatly and included reeds and

treelike plants with true roots and leaves.

Arthropods Plants appear to have colonized land in the company of

fungi. Even today, the roots of most plants are associated with

PROTEROZOIC PALEOZOIC fungi that aid in the absorption of water and minerals from
Ediacaran Cambrian
the soil (see Concept 31.1). These root fungi (or mycorrhizae),

695 635 605 575 545 515 485 0 in turn, obtain their organic nutrients from the plants. Such
Time (millions of years ago)
mutually beneficial associations of plants and fungi are evident

in some of the oldest fossilized plants, dating this relationship

VISUAL SKILLS Circle the branch point that represents the most recent back to the early spread of life onto land (Figure 25.12).
common ancestor of chordates and annelids. What is a minimum estimate of
Although many animal groups are now represented in
that ancestor’s age?
terrestrial environments, the most widespread and diverse

534 Unit four  Mechanisms of Evolution

Figure 25.12  An ancient symbiosis. This 405-million-year-old Figure 25.13  How speciation and extinction affect
fossil stem (cross section) documents mycorrhizae in the early land diversity. The species diversity of an evolutionary lineage will
plant Aglaophyton major. The inset shows an enlarged view of a cell increase when more new member species originate than are lost to
containing a branched fungal structure called an arbuscule; the fossil extinction. In this hypothetical example, by 2 million years ago both
arbuscule resembles those seen in plant cells today. lineage A and lineage B have given rise to four species, and no species
have become extinct. Over the next 2 million years, however, lineage
Zone of arbuscule- A experiences higher extinction rates than does lineage B (extinct
containing cells species are denoted by a dagger symbol, †). As a result, after 4 million
years (that is, by time 0), lineage A contains only one species, while
lineage B contains eight species.Lineage A

100 nm †

†

†

†

†

land animals are arthropods (particularly insects and spiders) Common Lineage B
and tetrapods. Arthropods were among the first animals to ancestor of
colonize land, roughly 450 million years ago. The earliest lineages A
tetrapods found in the fossil record lived about 365 million and B
years ago and appear to have evolved from a group of lobe-
finned fishes (see Concept 34.3). Tetrapods include humans, †
although we are late arrivals on the scene. The human lineage
diverged from other primates around 6–7 million years ago, 43210
and our species originated only about 195,000 years ago. If Millions of years ago
the clock of Earth’s history were rescaled to represent an hour,
humans appeared less than 0.2 second ago.

Interview with Geerat Vermeij: What fossils reveal about
the history of life

Concept Check 25.3 emerged from the sea, giving rise to several major new groups
of organisms. One of these, the amphibians, went on to dom-
1. The first appearance of free oxygen in the atmosphere inate life on land for 100 million years, until other tetrapods
likely triggered a massive wave of extinctions among the (including dinosaurs and, later, mammals) replaced them as
prokaryotes of the time. Why? the dominant terrestrial vertebrates.

2. What evidence supports the hypothesis that mitochondria The rise and fall of these and other major groups of organ-
preceded plastids in the evolution of eukaryotic cells? isms have shaped the history of life. Narrowing our focus, we
can also see that the rise or fall of any particular group is related
3. WHAT IF? What would a fossil record of life today to the speciation and extinction rates of its member species
look like? (Figure 25.13). Just as a population increases in size when
For suggested answers, see Appendix A. there are more births than deaths, the rise of a group of organ-
isms occurs when more new species are produced than are lost
Concept  25.4 to extinction. The reverse occurs when a group is in decline.
In the Scientific Skills Exercise, you will interpret data from
The rise and fall of groups the fossil record about changes in a group of snail species in the
of organisms reflect differences early Paleogene period. Such changes in the fates of groups of
in speciation and extinction rates organisms have been influenced by large-scale processes such
as plate tectonics, mass extinctions, and adaptive radiations.
From its beginnings, life on Earth has been marked by the rise
and fall of groups of organisms. Anaerobic prokaryotes origi-
nated, flourished, and then declined as the oxygen content of
the atmosphere rose. Billions of years later, the first tetrapods

chapter 25  The History of Life on Earth 535

Scientific Skills Exercise Species with
planktonic larvae
Estimating Quantitative Data from
a Graph and Developing Hypotheses Species with
nonplanktonic
Do Ecological Factors Affect
Evolutionary Rates? Researchers larvae
studied the fossil record to investigate
whether differing modes of larval dis- Paleocene Eocene
persal might explain species longevity 65 60
within one taxon of marine snails, the 55 50 45 40 35
family Volutidae. Some of the snail
species had nonplanktonic larvae: Millions of years ago (mya)
They developed directly into adults
without a swimming stage. Other species had planktonic larvae: For example, a bar that measures 1.1 cm on the graph represents a
They had a swimming stage and could disperse very long distances. persistence time of 1.1 cm * 3.6 my/cm = 4 million years.
The adults of these planktonic species tended to have broad geo- 2. Calculate the mean (average) persistence times for species with
graphic distributions, whereas nonplanktonic species tended to planktonic larvae and species with nonplanktonic larvae.
be more isolated. 3. Count the number of new species that form in each group begin-
ning at 60 mya (the first three species in each group were present
How the Research Was Done  The researchers studied the strati- around 64 mya, the first time period sampled, so we don’t know
graphic distribution of volutes in outcrops of sedimentary rocks when those species first appear in the fossil record).
located along North America’s Gulf coast. These rocks, which formed 4. Propose a hypothesis to explain the differences in longevity of
from 66 to 37 million years ago, early in the Paleogene period, are snail species with planktonic and nonplanktonic larvae.
an excellent source of well-preserved snail fossils. The researchers
were able to classify each fossil species of volute snail as having Data from  T. A. Hansen, Larval dispersal and species longevity in Lower Tertiary
planktonic or nonplanktonic larvae based on features of the earliest gastropods, Science 199:885–887 (1978). Reprinted with permission from AAAS.
formed whorls of the snail’s shell. Each bar in the graph shows
how long one species of snail persisted in the fossil record. Instructors: A version of this Scientific Skills Exercise can be
assigned in MasteringBiology.
Interpret the Data

1. You can estimate quantitative data (fairly precisely) from a graph.
The first step is to obtain a conversion factor by measuring along
an axis that has a scale. In this case, 25 million years (my; from 60 to
35 million years ago [mya] on the x-axis) is represented by a distance
of 7.0 cm. This yields a conversion factor (a ratio) of 25 my/7.0 cm =
3.6 my/cm. To estimate the time period represented by a horizontal
bar on this graph, measure the length of that bar in centimeters
and multiply that measurement by the conversion factor, 3.6 my/cm.

Plate Tectonics centimeters per year. They can also infer the past locations of

If photographs of Earth were taken from space every 10,000 the continents using the magnetic signal recorded in rocks
years and spliced together to make a movie, it would show
something many of us find hard to imagine: The seemingly at the time of their formation. This method works because
“rock solid” continents we live on move over time. Over the
past billion years, there have been three occasions (1 billion, as a continent shifts its position over time, the direction of
600 million, and 250 million years ago) when most of the
landmasses of Earth came together to form a supercontinent, magnetic north recorded Figure 25.14  Cutaway view
then later broke apart. Each time, this breakup yielded a dif- in its newly formed rocks of Earth. The thickness of the crust
ferent configuration of continents. Based on the directions also changes. is exaggerated here.
in which the continents are moving today, some geologists
have estimated that a new supercontinent will form roughly Earth’s major tec-
250 million years from now.
tonic plates are shown Crust
According to the theory of plate tectonics, the continents in Figure 25.15. Many
are part of great plates of Earth’s crust that essentially float
on the hot, underlying portion of the mantle (Figure 25.14). important geologic pro-
Movements in the mantle cause the plates to move over time
in a process called continental drift. Geologists can measure the cesses, including the
rate at which the plates are moving now, usually only a few
formation of mountains Mantle
536 Unit four  Mechanisms of Evolution and islands, occur at

plate boundaries. In some Outer
cases, two plates are core

moving away from each Inner
other, as are the North core
American and Eurasian

Figure 25.15  Earth’s major tectonic plates. The arrows indicate direction of movement. the tropics but has moved 40° to the
The reddish orange dots represent zones of violent tectonic activity. north over the last 200 million years.
When faced with the changes in climate
North Eurasian Plate that such shifts in position entail, organ-
American isms adapt, move to a new location,
Juan de Fuca Plate Arabian Philippine or become extinct (this last outcome
Plate Caribbean Plate Plate occurred for many organisms stranded
Plate Indian on Antarctica, which separated from
Plate Australia 40 million years ago).
Cocos Plate
Pacific Nazca South Continental drift also promotes allo-
Plate Plate American patric speciation on a grand scale. When
Plate supercontinents break apart, regions that
African Australian once were connected become isolated. As
Plate Plate the continents drifted apart over the last
200 million years, each became a sepa-
Scotia Plate Antarctic rate evolutionary arena, with lineages of
HHMI Animation: Plate Tectonics Plate plants and animals that diverged from
those on other continents.

plates, which are currently drifting apart at a rate of about Figure 25.16  The history of continental drift duringPresent
2 cm per year. In other cases, two plates are sliding past each the Phanerozoic eon.Cenozoic
other, forming regions where earthquakes are common.
California’s infamous San Andreas Fault is part of a border Earth’s youngest major
where two plates slide past each other. In still other cases, mountain range, the
two plates collide, producing violent upheavals and forming Himalayas, began to
new mountains along the plate boundaries. One spectacular form when India col-
example of this occurred 45 million years ago, when the lided with Eurasia
Indian plate crashed into the Eurasian plate, starting the for- about 45 million years
mation of the Himalayan mountains. ago. The continents
continue to drift today.
HHMI Video: Animated Life: Pangea, Wegener,
and Continental Drift 66 North America Eurasia By the end of the
Africa Mesozoic, Laurasia
Consequences of Continental Drift and Gondwana
SAomuethricAantarcMtiacdaaIngdasAicaaurstralia separated into the
Plate movements rearrange geography slowly, but their present-day continents.
cumulative effects are dramatic. In addition to reshaping the
physical features of our planet, continental drift also has a Millions of years ago Laurasia By the mid-Mesozoic,
major impact on life on Earth. Mesozoic Pangaea split into
135 northern (Laurasia)
One reason for this is that continental drift alters the habi-
tats in which organisms live. Consider the changes shown in Gondwana and southern
Figure 25.16. About 250 million years ago, plate movements (Gondwana)
brought previously separated landmasses together into a super- landmasses.
continent named Pangaea. Ocean basins became deeper,
which drained shallow coastal seas. At that time, as now, most 252 Pangaea At the end of the
marine species inhabited shallow waters, and the formation of Paleozoic, all of
Pangaea destroyed much of that habitat. Pangaea’s interior was Paleozoic Earth’s landmasses
cold and dry, probably an even more severe environment than were joined in the
that of central Asia today. Overall, the formation of Pangaea supercontinent
greatly altered the physical environment and climate, which Pangaea.
drove some species to extinction and provided new opportuni-
ties for groups of organisms that survived the crisis. VISUAL SKILLS Is the Australian plate’s current direction of movement (see
Figure 25.15) similar to the direction it traveled over the past 66 million years?
Organisms are also affected by the climate change that
results when a continent shifts its location. The southern
tip of Labrador, Canada, for example, once was located in

chapter 25  The History of Life on Earth 537

Finally, continental drift can help explain puzzles about mass extinction, which defines the boundary between the

the geographic distribution of extinct organisms, such as why Paleozoic and Mesozoic eras (252 million years ago), claimed

fossils of the same species of Permian freshwater reptiles have about 96% of marine animal species and drastically altered

been discovered in both Brazil and the West African nation of life in the ocean. Terrestrial life was also affected. For exam-

Ghana. These two parts of the world, now separated by 3,000 ple, 8 out of 27 known orders of insects were wiped out. This

km of ocean, were joined together when these reptiles were mass extinction occurred in less than 500,000 years, possibly

living. Continental drift also explains much about the current in just a few thousand years—an instant in the context of

distributions of organisms, such as why Australian fauna and geologic time.

flora contrast so sharply with those of the rest of the world. The Permian mass extinction occurred during the most

Marsupial mammals fill ecological roles in Australia analogous extreme episode of volcanism in the past 500 million years.

to those filled by eutherians (placental mammals) on other Geologic data indicate that 1.6 million km2 (roughly half

continents (see Figure 22.18). Fossil evidence suggests that mar- the size of Western Europe) in Siberia was covered with lava

supials originated in what is now Asia and reached Australia via hundreds of meters thick. The eruptions are thought to have

South America and Antarctica while the continents were still produced enough carbon dioxide to warm the global climate

joined. The subsequent breakup of the southern continents set by an estimated 6°C, harming many temperature-sensitive

Australia “afloat,” like a giant raft of marsupials. In Australia, species. The rise in atmospheric CO2 levels would also have
marsupials diversified, and the few eutherians that lived there led to ocean acidification, thereby reducing the availability of

became extinct; on other continents, most marsupials became calcium carbonate, which is required by reef-building corals

extinct, and the eutherians diversified. and many shell-building species (see Figure 3.12). The explo-

Mass Extinctions sions would also have added nutrients such as phosphorus to
marine ecosystems, stimulating the growth of microorgan-

The fossil record shows that the overwhelming majority of isms. Upon their deaths, these microorganisms would have

species that ever lived are now extinct. A species may become provided food for bacterial decomposers. Bacteria use oxygen

extinct for many reasons. Its habitat may have been destroyed, as they decompose the bodies of dead organisms, thus causing

or its environment may have changed in a manner unfavorable oxygen concentrations to drop. This would have harmed

to the species. For example, if ocean tem-

peratures fall by even a few degrees, spe- Figure 25.17  Mass extinction and the diversity of life. The five generally recognized
cies that are otherwise well adapted may mass extinction events, indicated by red arrows, represent peaks in the extinction rate of marine
perish. Even if physical factors in the envi- animal families (red line and left vertical axis). These mass extinctions interrupted the overall increase,
ronment remain stable, biological factors over time, in the number of extant families of marine animals (blue line and right vertical axis).

may change—the origin of one species 1,100
can spell doom for another.

Although extinction occurs regularly, 25 1,000

at certain times disruptive changes to the 900
global environment have caused the rate

of extinction to increase dramatically. 20 800
Total extinction rate
The result is a mass extinction, in 15 (families per million years): 700
which large numbers of species become Number of extant families:600
extinct worldwide.

The “Big Five” Mass Extinction 10 500
Events 400

Five mass extinctions are documented in 5 300
the fossil record over the past 500 million 200
years (Figure 25.17). These events are 0 Paleozoic Mesozoic 100
particularly well documented for the deci- Era OS D Cenozoic 0
mation of hard-bodied animals that lived Period C C P Tr J C P NQ
in shallow seas, the organisms for which 66 0
the fossil record is most complete. In each 541 485 444 419 359 299 252 201 145
mass extinction, 50% or more of marine
species became extinct. Time (millions of years ago)

Two mass extinctions—the Permian INTERPRET THE DATA As mentioned in the text, 96% of marine animal species became extinct in the
and the Cretaceous—have received Permian mass extinction. Explain why the blue curve shows only a 50% drop at that time.
the most attention. The Permian

538 Unit four  Mechanisms of Evolution

oxygen-breathers and promoted the growth of anaerobic bac- The crater is the right size to have been caused by an object
teria that emit a poisonous metabolic by-product, hydrogen with a diameter of 10 km. Critical evaluation of this and other
sulfide (H2S) gas. Overall, the volcanic eruptions may have hypotheses for mass extinctions continues.
triggered a series of catastrophic events that together resulted
in the Permian mass extinction. HHMI Video: The Day the Mesozoic Died

The Cretaceous mass extinction occurred 66 million years Is a Sixth Mass Extinction Under Way?
ago. This event extinguished more than half of all marine
species and eliminated many families of terrestrial plants As you will read further in Concept 56.1, human actions,
and animals, including all dinosaurs (except birds, which are such as habitat destruction, are modifying the global environ-
members of the same group; see Figure 34.25). One clue to ment to such an extent that many species are threatened with
a possible cause of the Cretaceous mass extinction is a thin extinction. More than 1,000 species have become extinct
layer of clay enriched in iridium that dates to the time of in the last 400 years. Scientists estimate that this rate is 100
the mass extinction. Iridium is an element that is very rare to 1,000 times the typical background rate seen in the fossil
on Earth but common in many of the meteorites and other record. Is a sixth mass extinction now in progress?
extraterrestrial objects that occasionally fall to Earth. As a
result, researchers proposed that this clay is fallout from a This question is difficult to answer, in part because it is
huge cloud of debris that billowed into the atmosphere when hard to document the total number of extinctions occur-
an asteroid or large comet collided with Earth. This cloud ring today. Tropical rain forests, for example, harbor many
would have blocked sunlight and severely disturbed the undiscovered species. As a result, destroying tropical forest
global climate for several months. may drive species to extinction before we even learn of their
existence. Such uncertainties make it hard to assess the full
Is there evidence of such an asteroid or comet? Research extent of the current extinction crisis. Even so, it is clear that
has focused on the Chicxulub crater, a 66-million-year-old losses to date have not reached those of the “big five” mass
scar beneath sediments off the coast of Mexico (Figure 25.18). extinctions, in which large percentages of Earth’s species
became extinct. This does not in any way discount the seri-
Figure 25.18  A trauma for Cretaceous life. Beneath the ousness of today’s situation. Monitoring programs show that
many species are declining at an alarming rate due to habitat
Caribbean Sea, the 66-million-year-old Chicxulub crater measures loss, introduced species, overharvesting, and other factors.
Recent studies on a variety of organisms, including lizards,
180 km across. The horseshoe shape of the crater and the pattern of pine trees, and polar bears, suggest that climate change may
hasten some of these declines. The fossil record also high-
debris in sedimentary rocks indicate that an asteroid or comet struck at lights the potential importance of climate change: Over
the last 500 million years, extinction rates have tended to
a low angle from the southeast. This drawing represents the impact and increase when global temperatures were high (Figure 25.19).

its immediate effect: a cloud of hot Figure 25.19  Fossil extinctions and temperature. Extinction
rates increased when global temperatures were high. Temperatures
vapor and debris that could have were estimated using ratios of oxygen isotopes and converted to an
index in which 0 is the overall average temperature.
killed many of the plants and animals NORTH
3 Mass extinctions
in North America within hours. AMERICA

Yucatán Chicxulub
Peninsula crater

Relative extinction rate of marine animal genera 2

1

0

–1

–2 –3 –2 –1 0 1 2 3 4

Cooler Warmer

Relative temperature

chapter 25  The History of Life on Earth 539